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In the preceding chapter, the complex environment in which roots grow and carry on their several activities has been discussed. Now, the structure of roots will be examined somewhat in detail as well as their adaptation to live in the soil and to absorb the water and nutrients necessary for plant growth.
The roots of cultivated plants are underground parts that spread widely through the soil and absorb water and mineral matter which they conduct to the stems and leaves. They also give firm anchorage to the stem. In addition, they frequently accumulate reserve foods and, in some plants, serve as organs of reproduction.
When a seed germinates, the primary root develops at the lower end of the tiny stem of the embryo plant. Very soon, lateral roots begin to appear which, with their branches, greatly increase the absorbing area and anchoring power of the root. Many cereals, such as wheat, oats, and corn, usually have three roots arising from the seed (seminal roots), i.e., the primary root and two almost equally large laterals. These with their branches (sometimes supplemented by other primary roots) constitute the primary root system. The remaining portion of the root system arises from the nodes or joints of the stem in the soil. Frequently, as in the brace or prop roots of corn, they grow from nodes above the soil surface. They are unbranched aboveground and are covered with a gummy substance which prevents drying out, but upon entering the soil, they branch and rebranch quite like other roots. Roots not arising from the seed or as branches of seed roots but from stems or leaves are called adventitious. In the case of the cereals and other grasses which have strong, threadlike or fibrous roots, the larger part of the root system is composed of the adventitious roots which collectively make up the secondary root system. It is worthy of note that the roots of the secondary system originate only about an inch below the soil surface, even if the grain is planted 2 to 3 inches deep (Fig. 5).
Fig. 5.--White Kherson oat plant, showing the primary and secondary root systems.
A very different type of root system is found in such plants as clover, sugar beet, sunflower, and alfalfa. The germinating seed develops the primary root, which pushes straight downward and is usually branched throughout its entire extent. Thus, the root system of such plants as beets and sunflowers consists of one main root, the taproot, with its many branches. There is no secondary root system of adventitious roots as among the cereals (Fig. 6). In the case of certain clovers which propagate by runners, the roots of the new plants arising from these are entirely adventitious.
Fig. 6.--Alfalfa seedling, illustrating the taproot system.
Internally, the roots of all cultivated plants are built on the same general plan and differ from one another only in detail. Although the growing root ends are usually only about a millimeter in diameter, they show a wonderful differentiation and are remarkably adapted to perform their several functions (Fig. 7).
Fig. 7.--Enlarged view of the end of a root, showing root cap, region of elongation, and root hairs. (After Transeau, General Botany, The World Book Company.)
Zone of Division.--Examination under the microscope of a thin section cut longitudinally through the end of a root shows that in the growing tip, the units of structure, the cells, are very much alike. They are very minute in size, quite cubical in shape, very thin walled, and filled with the living substance called protoplasm. These groups of cells, which are found in all growing points, produce new cells by division. Indeed, it is for this reason that the tissue is called meristem (Greek, to divide). The very tip end of the root which they form is called the zone of division (Fig. 8).
Fig. 8.--Longisection of a root of corn, showing root cap and zone of division, zone of elongation, and zone of maturation. A single meristern cell and a single parenchyma cell are greatly enlarged.
Zone of Elongation.--A short distance behind the root tip, usually only a millimeter or two, the cells cease dividing and greatly increase in size. Although they grow slightly in all three dimensions, the principal direction of enlargement is parallel with the axis of the root. Hence, the formerly cubical cells (square in longisection) now appear rectangular in shape. The elongation of these cells is so marked and takes place so rapidly that the portion of the root which they make up, just back of the zone of division, is called the zone of elongation. Close examination shows that the cell walls are slightly thicker than before, that the protoplasm no longer fills the cell but is usually confined to a thin layer lining the several walls, and that the center of the cell is filled with a tiny droplet of cell sap, i.e., water and substances dissolved or dispersed therein. The tissue which these soft, watery cells compose is called parenchyma.
The Root Cap.--It is obvious that the elongation of the parenchyma cells pushes the root tip further into the soil. The delicate meristematic cells of which it is composed are protected from actual contact with the soil by means of the root cap, the calyptra. This covering of cells, of which the outermost ones are dead, envelops and protects the growing tip very much as a thimble protects the finger. It extends back over the root for a distance usually of about a millimeter. The calyptra arises from the lowermost cells of the growing point. As the root-cap cells mature, they become parenchyma and are constantly pushed out by the addition of new cells from within. Those on the outside next to the soil are more or less flattened by pressure from within and slough off as they rub against soil particles when the root grows deeper. These cells, by their slimy nature, lubricate the tip. But throughout the life of the root, even if it penetrates many feet into the soil, the root cap is so constantly renewed by new cells from within that the delicate tip is actually pushed through the soil without, as it were, coming in contact with it. It should be kept clearly in mind that the zone of elongation is very close to the root end and that only about a millimeter or less of the root is pushed ahead. This explains why roots can penetrate even into stiff clay soils without buckling as would inevitably be the case were the zone of elongation further from the tip. Moreover, the root is not forced through the soil as one might push down a steel wire, but its direction of growth is determined by responses to several stimuli.
Gravity is the main stimulus in directing the downward course of the primary root. But it also responds to the stimulus of contact with solid bodies and may deviate from its course in passing around large soil particles or through compacted soils. Differences in the amount of moisture also induce curvatures, the roots being attracted towards moist soil areas. Similar positive responses are shown by growth towards more favorable oxygen supplies. Under such influences, the root tip pursues a general downward course through the soil, its slight movement from side to side aiding it in penetrating the firm substratum. The force developed by a growing root is surprisingly great. Careful measurements show that it is frequently 100 pounds or more to the square inch.
Zone of Maturity: Origin, Arrangement, and Functions of Mature Tissues.--Turning attention again to the portion designated as the zone of elongation, the beginnings of three regions soon become very distinct. The outermost layer of cells which envelops the cylindrical root is rapidly differentiated into an epidermis. The central core of tissue which will form the stele is already distinguishable from the cylindrical mass of cells (the formative cortex) which surrounds it and fills in between the stele and the epidermis. This ensheathing mass of cells becomes the cortex. In this partly differentiated state, these regions of developing cells of different sizes and shapes are called technically the dermatogen (Greek, skin producing), plerome (Greek, that which fills in), and periblem (Greek, to clothe), respectively.
Following elongation and enlargement, the cells gradually mature, some groups specializing as absorbers of water and nutrients, others as carriers of water or food. Some store food, others add tensile strength for anchorage, while a few retain their capacity for further growth. The development of these various, tissues in order properly to carry on their activities is very interesting. Their position in the root is best seen in cross-section (Fig. 9), although the longisectional view is quite essential for an understanding of their development (Fig. 8). Such sections beyond the zone of elongation and in the zone of maturation and maturity show that many of the epidermal cells have developed root hairs. These cylindrical, protoplasmic-lined protuberances, about 2 to 8 millimeters in length, arise by the outward extension and growth of the outer walls of some of the epidermal cells. Their presence may increase many fold the absorbing surface of the root, i.e., the part in contact with soil.
Fig. 9.--Diagrammatic cross-section of a young dicotyledonous root through the root-hair zone.
Just beneath the epidermis is the outermost layer of the cortex. One of its functions is the replacement of the epidermis when it is destroyed. The somewhat cylindrical cells of the cortex (more or less circular in cross-section) have arisen from rows of elongated cells, which, turgid with cell sap, have pulled apart at the corners after a splitting of the three layered wall. the central, rather gelatinous lamella of the three The spaces between the cells, designated as intercellular spaces, furnish. an excellent aerating system, which is very necessary since all living cells need means of securing oxygen and of disposing of the waste carbon dioxide arising through respiration. Water and solutes absorbed by the root hairs and other portions of the epidermis must pass through these cells on their way to the main highways of sap movement, the tubes in the stele.
In the central cylinder or stele, longitudinal rows of parenchyma cells develop into tubes, first, by dissolving of their end walls (a phenomenon due to digestive enzymes); then by a strengthening of the remaining walls by the addition of wooden (lignified) rings and spirals within; and finally by the death of the protoplasm, The function of these tubes is clearly indicated by the fact that they arise within the stele at the same place along the root where the root hairs develop from the epidermis without. Of course, they are continuous with similar tubes in the stems and leaves. Very large tubes are soon produced which have their inner walls partially thickened with a woody network or completely thickened and lignified except for small pits which permit the exit of water necessary to supply living tissues along these highways to the leaves. Other conducting elements, consisting usually of single, elongated, and variously pitted cells called tracheids, may occur. The arrangement of the ringed, spiral, etc., tracheary tubes (so called because of their resemblance to tracheae in higher animals) in radii directed towards the center of the root is characteristic and, as will be seen, very significant. As a rule, these strands meet at the center of the root, especially in dicotyledonous plants.
Occupying a portion of the area between the radiating tracheary tubes are groups of food-conducting cells which transport elaborated foods, such as proteins, etc., from the aboveground parts downward. These, too, have been formed from rows of elongated parenchyma cells but with less modification. In some, the end walls have been dissolved, but only in places, giving the thickened partition between cells somewhat the appearance of a sieve; hence, the name sieve tissue. Protoplasm seems essential for food conduction, and the sieve cells, which form long uninterrupted food highways from the leaves to the roots, contain living protoplasm. Much of this food diffuses downward to feed the vigorously growing (dividing and elongating) cells below. Some food, however, is transported across the cortex to the epidermis and root hairs, but nearly always a part is stored in the cortical cells. Hence, the prominent activities of the cortex are transport and storage.
Bast fibers usually accompany the elongated parienchyma cells and sieve tubes, and occupy a position just outside of them in the stele. They are much elongated cells with very thick walls and small cavities and, when fully grown, are destitute of protoplasm. Like tracheary tubes, they function better dead than alive. The pointed ends of the cells are firmly dovetailed together, and the bast fibers and wooden vessels add much pulling or tensile strength. It should be recalled that roots are exposed only to pulling strains in resistance to which the arrangement of the tough tissues in the center, like the solid cable, is best.
The arrangement of the tracheary tubes or wood (xylem) and sieve and bast fibers (or phloem) in separate groups prevents a mixing of the stream of water and inorganic salts, diffusing inward and upward, with the stream of food materials carried downward in the phloem. In fact, the mature endodermis, which is the innermost layer of the cortex, is characterized, at least in many plants, by the presence of a layer of cork or suberin which is usually deposited all over the inner surface of the cells. This wall layer renders the endodermis relatively very impermeable to both water and solutes, except through certain unsuberized cells which lie opposite the rays of xylem strands. 154 Through these passage cells, the water and solutes enter directly into the tracheary tubes.
Fig. 10.--Longisection of a root, showing the origin of a lateral. The formative regions are labeled according to mature regions into which they develop.
Origin and Development of Lateral Roots. Entirely surrounding the stele and forming its outermost portion are one or more layers of cells, the pericycle; which cells have the power of division. It is from this layer that lateral roots nearly always take their origin. By means of repeated division and growth of these cells, a mass of cells or a tissue is formed in which a growing point and root cap are soon differentiated (Figs. 10, 11). The innermost layer of the cortex, the endodermis or starch sheath, adjoining the pericycle on the outside, helps furnish food for the actively growing rootlet which, by the secretion of digestive enzymes, dissolves the tissue of the cortex and emerges into the soil. The laterals burst through the cortical tissue and appear in a definite number of rows corresponding to the number of groups of woody vessels first formed, opposite which they originate. Thus, the number of rows of branches in a sugar beet. is two, and in a pea, ordinarily four. Their position favors the direct transfer of water to the conducting tissues of the main root.
Fig. 11.--Seedling of the white lupine (Lupinus albus) made semitransparent by placing it for several hours in a weak aqueous solution of potassium hydroxide. Note the lateral roots of different ages. (After Gager, Fundamentals of Botany, P. Blakiston's Son & Co.)
Structurally, the lateral roots are like the main root from which they arise. Some of the sieve tubes of the old root connect directly with those of the laterals, and water and solutes absorbed by the laterals are transferred through tracheary tubes which join directly the main upward xylem strands. Lateral roots, of course, do not emerge into the soil until elongation has ceased in that part of the main root in which they arise. Otherwise, they would be torn off.
Root branches arise very irregularly, unlike stems and leaves, for there is no division of roots into nodes or joints and internodes as occurs in stems. Lateral roots begin to appear only a few days after germination. Their direction of growth is frequently at a more or less definite angle with the main root. That their position is influenced to a considerable degree by the main root may be shown by the removal of the tip of the latter when one or more of the side roots curve downward and pursue the course usually taken by the main root. Branches from the laterals extend out in all directions, quite as frequently towards the soil surface as away from it. Thus, the soil comes to be more thoroughly occupied.
ROOT HAIRS AND FACTORS AFFECTING THEIR DEVELOPMENT
Root hairs are of such importance in absorption that they warrant further consideration. 191 All of the materials that enter the plant from the soil must first pass through the walls of the root hairs or other epidermal cells as well as the layer of cytoplasm lining them. Although they are so small that it often requires 70 or more laid side by side to equal a millimeter, they occur in such great numbers (frequently 200 to 300 per square millimeter of epidermal area) that they greatly increase the absorbing surface. For example, in the case of corn, it has been calculated that the increase is 5.5 times that of a hairless root of similar area; in garden peas, 12.4 times, and in certain other plants, as much as 18 times. 180
Fig. 12.--Tip of a root hair with adhering particles of soil, enlarged about 240 times. (Redrawn after Strasburger et al, A Textbook of Botany, The Macmillan Company.)
Root hairs grow usually at right angles to the root and extend out through the moist air of the soil interspaces until they strike solid particles. Here, they spread over these particles and frequently more or less completely surround them (Fig. 12). The outer lamella of the root-hair wall is composed of pectic materials which become mucilaginous. The intimate contact of root hairs with the water and solutes that form a film around the soil particles is due to the presence of mucilaginous material 102 which, in some plants, has been shown to be pectin mucilage; 161 hence, the high efficiency of the root hairs as absorbing organs. In fact, the delicate root hairs are so tender and so firmly attached to soil particles that it is practically impossible to remove the roots from the soil without injuring or tearing off many of them. Only a few seconds exposure to dry air causes them to wither and die. This explains why transplanted plants frequently wilt. They revive usually only after new root hairs are formed.
Root hairs do not cover the entire root system but are limited to the younger portions, except the zones of division and elongation. The root-hair zone appears as if covered with a white fuzzy coating and, though often only a few millimeters long, may extend through distances of many inches, especially on rapidly growing plants in moist soil. The life of root hairs is frequently short. They may function for only a few days, but often they remain active for several weeks and in certain plants for more than a year. 228, 136 In one experiment, nearly all of the roots of corn plants in the eighth-leaf stage were found to retain root hairs, which were apparently functioning, over most of their area. On the roots of winter wheat, the piliferous zone may extend for 2 feet along the roots of the primary root system, if soil conditions are very favorable for their development.
As roots grow longer, new hairs are constantly formed just back of the zone of elongation (where they will not be torn off by growth), while the older hairs at the upper end of this zone die and slough off. Thus the root-hair zone advances as the roots grow further into the soil. This so-called migration of the root-hair zone constantly brings new root hairs into contact with new soil particles. In this way, fresh stores of water and solutes are constantly being made available to the growing plant, whose roots like the tops, continue growth until the crop is mature. Moreover, it is an advantage to the plant because these new soil regions are free from deleterious substances which may accumulate in the region of the old root hairs. Materials from dead root hairs, old root-cap cells, or organic substances washed into the soils by rains, may become altered by the action of microorganisms so as to produce poisons.
The amount of water and air in the soil has a marked effect upon the development of root hairs. The roots of cultivated crops usually produce few root hairs in wet soil. In moderately moist soil corn roots are almost woolly with root hairs, but there are fewer in wet soil, and usually none in water. The same general relation's hold for most cultivated plants, although there are some exceptions. Wet soils contain less air than dry ones and a good oxygen supply seems necessary to promote abundant development of root hairs, at least in many species. Most cultivated plants require a well-aerated soil. Indeed, one of the chief advantages of stirring the soil is to admit air to the roots. In cultivated plants, so long as they do not wilt, it has been observed that the most abundant production of root hairs takes place at a water content somewhat less than that which will afford the highest yield. Of course, if soils become very dry, both root hairs and young rootlets die. Root-hair development may also be retarded by a very concentrated soil solution such as occurs in alkali soils. Extremes of temperature are also inimical to their growth. 105a They develop in the light and dark about equally well, provided there is ample moisture. The great importance of roots hairs may be realized when it is found that absorption is practically limited to the root-hair zone.
Back of the piliferous zone, when the root hairs die, the external cortical cells may become suberized or corky; i.e., the cell walls are infiltrated with fatty, waterproof substances, and the root, while continuing its work of conduction, ceases to absorb. In the roots of long-lived cultivated plants, like clover and alfalfa, growth in diameter is brought about by the formation of a cylinder of meristem tissue, the cambium. Certain of the cells lying adjacent to the xylem and phloem strands, including those of the, pericycle lying just outside the xylem, begin to divide. Although, at first, the cambium as viewed in cross- section is not circular, it soon takes on the circular form (Fig. 13). New (secondary) xylem elements (tracheary tubes, tracheids, wood fibers, and wood parenchyma) arise from the inside of the cambium opposite the phloem strands. At the same time, new (secondary) phloem elements (sieve tubes, bast fibers, and parenchyma) are formed just inside the old phloem. This results in a circle of fibrovascular bundles each separated into xylem (inner) and phloem (outer) portions by a layer of meristem. This cambium also cuts across the rows of parenchyma cells (medullary or pith rays) which have, been formed opposite the old xylem strands and thus completes the cylinder. New wood (xylem) and new bark (phloem) are added each year.
Fig. 13.--Diagrams showing stages in the secondary increase in thickness of a root: A, before the appearance of cambium; B, the formation of the cambium ring; C and D, stages in the development and growth of secondary phloem and xylem. Secondary increase in thickness due to the activity of phellogen is not shown. (Reprinted by Permission, from A Textbook of Botany by Holman and Robbins, Published by John Wiley and Sons, Inc.)
While these changes are taking place in the stele and before the epidermis is stretched to the breaking point by this increase of cells within, certain cells of the pericycle undergo repeated division, and thus another cambium is formed. This cambium, which lies outside the one passing through the bundles, gives rise towards the outside to cork cells, i.e., layer after layer of brick-shaped cells with walls infiltrated with fatty substances called suberin. For this reason, it is called cork cambium or phellogen. All of the tissues outside the pericycle are thus deprived of water and food and soon die. A secondary cortex is produced, however, just within the phellogen, as a result of its repeated division. The bark of old roots, as in stems, includes all the tissues outside the cambium which lies between the phloem and xylem.
In fleshy roots, such as those of the beet, the several rings or bands are due to the origin and growth of several vascular (not cork) cambium cylinders arising, one after another, outside the original cambium. The number varies with the length of the growing season. These cambium rings give rise to narrow bands of tissue which contain small amounts of both xylem and phloem elements, and large amounts of parenchyma tissue in which quantities of food and water are stored. Other familiar examples of fleshy roots are turnips, sweet potatoes, carrots, parsnips, and radishes. Reserves of food and water are very advantageous to plants during periods of drought.
Thus, it comes about that only the less conspicuous and younger portions of the root system are really active in absorption. The older parts have the walls of their exterior cells more or less thoroughly waterproofed and their functions are almost entirely those of anchorage, transport, and storage.
The rapidity of root growth is quite as remarkable as root extent. In many of our common grasses, the rate of root elongation is over half an inch per day (Fig. 14).
Fig. 14.--Root development of tall marsh grass (Spartina michauxiana) at the age of 11 weeks.
Roots of the primary system of winter wheat have been found to grow at a similar average rate over a period of 70 days (Fig. 15).
Fig. 15.--Diagram showing the rate of growth of the primary root system of winter wheat (cf. Fig. 70).
The widely spreading roots of potatoes, when they begin their vertical descent, may elongate at the rate of 1 inch a day for a period of 2 weeks or more. When the main vertical roots of corn begin to develop, they sometimes penetrate downward, under exceptionally favorable conditions, at the remarkable rate of 2 to 2.5 inches per day during a period of 3 or 4 weeks. A similar growth rate has been determined for the horizontal roots of squash.
The great extent of roots in relation to aboveground parts is often very striking. For example, a honey locust seedling (Gleditsia triacanthos), 13 weeks after seed germination, although reaching a height of only 9 inches, produced a very widely spreading-root system that extended well into the fourth foot of soil (Fig. 16). Maize has a wonderfully developed root system which occupies rather thoroughly over 200 cubic feet of soil.
Fig. 16.--Honey locust (Gleditsia triacanthos) seedling about 3 months old.
Many data are available on the ratio of dry weight of roots to tops, but they are difficult to interpret, since the roots are nearly always much more finely divided than are the above-ground parts and, consequently, have a greater surface. Moreover, in terms of function, the larger, thicker, and heavier roots are least significant, and the delicate branchlets, too often lost in such determinations, are of greatest importance, although adding little in weight. In the case of corn, for example, the dry weight of the roots is only 7 to 8 per cent of the dry weight of the tops. 126 Undoubtedly, the main fibrous roots make up the larger portion of the root weight. But, as is shown below they constitute only about 11 per cent of the entire absorbing area. These differences are even more striking in the case of plants with thick taproots, like red clover or alfalfa. Similar objections hold true in stating root extent in terms of length.
Unquestionably, the best method of comparison is that of absorbing area, but because of the difficulty of recovering the root system from the soil in its entirety and the onerous task of measuring the length and diameter of all its parts, few data are available. Corn grown for 5 weeks in rich loess soil with a nearly optimum water content produced 19 main roots with 1,462 branches of the first order. Upon these, there were 3,221 branches of the second and third order, only three of the secondary branches being furnished with laterals. The main roots made up only 11 per cent of the total absorbing area, which was 1,183 square centimeters; 75 per cent of the area was furnished by the primary laterals and the remaining 14 per cent by the branches from these. Plants of similar age and approximately the same size (eighth-leaf stage) grown in a moist sandy loam had 23 main roots which furnished 10 per cent of the 2,262 square centimeters of area. The 1,795 laterals of the first order made up 45 per cent, and the 8,427 laterals of the second order (no further branching occurring), the remaining 45 per cent of the absorbing surface.
In the example just cited, the main roots of corn constituted only 4 or 5 per cent of the total length; the primary branches, 47 to 67 per cent, depending upon the conditions under which the plants. were grown; and the finer secondary and tertiary laterals, which are very difficult to recover, from 29 to 48 per cent of the total.. Two-months-old alfalfa plants, grown in a rich, rather wet, silt loam, had a root area of 85 square centimeters, which was 15 per cent less than that of the tops. The taproot furnished 21 per cent of this area, the primary branches 41 per cent, the secondary branches 35 per cent, and the tertiary branches the remaining 3 per cent. These measurements, which do not include the increased area due to root hairs, give a fair idea of the great importance of the finer branches in securing water and nutrients utilized in crop growth.
Roots have a form and structure remarkably adapted to perform their functions of anchorage; and of absorption, conduction, and storage of water, nutrients, and elaborated foods. As regards origin, the primary root, developing from the lower end of the stem of the embryo, with its branches, may constitute the entire root system, as in beets and many other dicotyledons. But in monocotyledons, like the cereals, as well as in certain other plants, this primary root system is supplemented by a secondary one which consists of branches arising adventitiously from the basal nodes of the stem. An examination of root structure shows that each part or tissue has its special activities to perform. The growing tip of meristem in the zone of division continually produces new cells. These enlarge, chiefly in one direction, just back of the tip in the zone of elongation, and push the growing tip, protected and lubricated by the root cap, into the soil with great force. Its pathway is, in the main, determined by gravity but is also affected by water, air, and other stimuli. The maturing root shows three well-defined regions: epidermis, cortex, and stele or central cylinder. The permeable epidermis, with surface greatly increased by root hairs, absorbs water and nutrients. The abundance of root hairs varies greatly with water content and air supply; there are few or none in very wet soil. The cortex is chiefly active in lateral transport of water and nutrients and in food storage. The tracheary tubes of the stele conduct absorbed materials upward to the green stems and leaves where they, with carbon dioxide from the air, are made into plant foods. These foods, in a soluble form, are transported downward along the phloem (sieve tubes, parenchyma) highways and nourish the growing root. Groups of bast fibers furnish flexibility and tensile strength, the former undoubtedly being increased by the rather spongy parenchyma tissue of the cortex. The pericycle gives rise to buds which develop into lateral roots having a structure identical with that of the parent, and in these, the process of branching is again repeated. The roots and branches extend rapidly in all directions, filling many cubic feet of soil. They are so numerous that their combined area often exceeds that of the shoot. Since the epidermis of older roots sloughs off as layers of cork are formed within, absorption is confined to the younger parts. By the formation of a cambium, the power of continued annual growth is provided in many long-lived plants.