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The eggplant (Solanum melongena esculentum) is a rather coarse, erect, annual, branching herb. It reaches a height of 2 to 4 feet and becomes somewhat woody. Like its relative, the tomato, it is grown for its fruit. Eggplant requires a high temperature for its best growth, and consequently it is cultivated commercially mainly in the South. It is rather a common crop in the home garden throughout much of the United States, how ever. Like tomatoes, eggplants are usually transplanted into the garden but this must be done in such a manner that the root system is not disturbed to an extent which will check the growth of the plant. Hence, they are transplanted at least once, often into pots, boxes, or bands, from which the whole soil-root mass may be set into the field with a minimum of injury to the roots.
In the following studies plants were grown from seed sown in the field on June 2. Seed of the Black Beauty variety was used. The plants were spaced 2.5 feet apart in rows 3 feet distant.
Early Development.--Owing to dry weather the plants grew slowly and on July 24 they were only 6 inches tall. Each plant had six or seven leaves in addition to the cotyledons which were still green. The leaf blades were about 3.5 by 2 inches in greatest dimensions although the largest were 5 by 4 inches and thus presented considerable surface.
The underground parts were characterized by a strong taproot which, with its deeper branches, furnished the major absorbing area. In addition, a large number of more superficial, horizontally spreading roots ramified the surface soil. A lateral spread of 18 inches and a maximum depth of 34 inches had been attained (Fig. 77).
Fig. 77.--Eggplant of the Black Beauty variety showing the root extent at the age of about 7 weeks.
The surface inch of taproot, which was 7 millimeters thick, was free from branches but the next inch had 30 simple rootlets 0.5 millimeter thick and about 0.3 inch long. Twenty were found on the third inch of taproot. Like the others, they often occurred in clusters or clumps. Among these were a few longer, rebranched roots. Below this to a depth of 2 feet branches originated at the usual rate of 5 to 9 per inch. Those in the surface 6 inches ran horizontally for distances of 3 to 18 inches. They were branched at the rate of 5 to 8 laterals per inch, the branches being irregularly grouped and varying from 0.3 to 2 inches in length. A few were longer (8 to 11 inches) and usually pentreated more or less vertically downward. These were quite well rebranched. In the next 8 inches the branches were shorter but at still greater depths numerous long ones occurred. Their course was horizontal or obliquely downward for a short distance, usually 1 to 3 inches, and then more vertically downward. As a whole only the longer, deeper roots were as well branched as those in the shallower soil. In fact many of the deeper, rather horizontal, shorter laterals were poorly furnished with branchlets. The very sinuous course of the taproot and of many of the smaller roots was marked. Considering the size of tops the root system was well developed.
Mature Plants.--A second and final examination was made Oct. 3. As a result of favorable moisture conditions late in the summer the plants had made a good growth. They averaged 33 inches in height and the tops had a spread of 20 inches. Plants of average size were used for root studies. Each plant had 5 to 7 branches and 75 leaves. The leaf blades averaged 8 inches in length and 5 inches in width, thus exposing a very large transpiring surface. The plants were fruiting abundantly. The fruits ranged from only 1 inch to 5 inches in diameter. The small size was due to the retarded growth during the dry weather of early summer.
The root system had made a most marked development. The strong taproot was approximately 1 inch in diameter near the soil surface. It tapered to 4 millimeters in thickness at the 12-inch level but like many of its large laterals was 2 millimeters in diameter throughout much of its tortuous course. Due to density of roots and the great water loss by the tops, the soil in this plat was quite dry and hard. This condition was reflected in the crooked course pursued by most of the roots, short zigzags and kinks through distances of only an inch or two being characteristic.
That the surface 12 inches of soil was overcrowded with roots should be clear when it is stated that a single plant had 275 smaller main roots in this soil area as well as 18 larger ones. Most of these roots, which were only 1 millimeter or less in diameter, extended 8 to 14 inches horizontally from the plant and ended or turned downward for a few inches. No roots were found at greater distances than 2 feet. All were branched very profusely to the second and third order and formed dense root masses. Some were yellowish brown and showed decay.
Of the 18 larger roots originating in the surface 15 inches of soil (and all but 2 in the surface 10 inches) nearly all ran horizontally, or only slightly obliquely downward, for distances seldom exceeding 2 feet. Then, turning downward, they ran vertically or again obliquely to depths of 4 to 7 feet.
A fair idea of the general behavior of the main roots may be gained from the following example. One lateral 5 millimeters in diameter originated at a depth of 3.5 inches. It ran almost horizontally so that when 16 inches long it was only 7 inches deep. Then it turned abruptly downward after giving rise to 4 rather horizontal branches. These extended outward and upward 19 to 24 inches further and ended within 2 to 4 inches of the soil surface. The large main root continued its downward course but gave rise to no large branches until it reached the 18-inch soil level. Here 7 rather equal branches about 1 millimeter in diameter arose. The main root continued its vertically downward course to an ultimate depth of 6.8 feet. The branches spread 2 to 18 inches on all sides and then turned downward. This system of 8 roots and their network of branches completely ramified the fourth, fifth, and sixth foot of soil at least for a distance of 8 inches or more on all sides of the main root. Horizontal and vertical branches were very profuse in the joints of the clay, filling if with a white, glistening network. The smallest branches were hairlike.
It is difficult to visualize this intricate network of roots in the deeper soil. It should be pointed out, however, that many of the roots reached depths of 5.5 to 7 feet; that numerous long branches greatly increased the absorbing territory; and that both main and subsidiary roots were clothed with laterals 0.5 to 5 inches long at the rate of five to eight per inch, the longer ones being rebranched.
The mass of absorbing roots in the surface soil can scarcely be overdrawn. In contrast the well-ramified deeper soil seemed rather poorly occupied. The small branches from the horizontal portions of these major laterals were often 8 to 12 inches long. As many as 14 frequently arose from a single inch of root length. They were extremely well rebranched, forming a dense root network to the very soil surface and indeed throughout the 12 inches of the richest surface soil. The ultimate branchlets averaged 7 per inch and were 0.2 to 1 inch long.
The taproot frequently divided at depths of 15 to 18 inches where the hardest soil occurred. Even near its end, or the end of one of its forks, it sometimes divided into a whole cluster of fine rootlets. Although somewhat poorly branched throughout, on certain portions of its course small branches occurred nearly to the tip.
The total absorbing surface of such a root system must be very large indeed. Just how much of it is actively functioning late in the life of the plant, i.e., during flower and fruit production, is a problem of considerable difficulty but of great physiological importance.
Summary.--Eggplants grown from seed are only 6 inches high when 7 weeks old. The strong taproot has penetrated nearly to the 3-foot level. Numerous horizontal branches spread 12 to 18 inches in the surface 3 to 8 inches of soil. Other branches, some 2 feet long and running obliquely downward, arise from the deeper portions of the taproot, spread 6 to 8 inches on all sides of it, and furnish the larger absorbing area. On mature plants the root system is very extensive. Nearly 300 roots often arise in the surface foot of soil. Most of these merely ramify the soil 8 to 14 inches from the plant. But 15 or more stronger laterals usually run outward 1 to 2 feet and then penetrate downward 4 to 7 feet. Their long, horizontal branches extend the lateral spread to 4 feet. Even in the deeper soil a lateral spread of 3.5 feet is common. The absorbing system in the surface soil is quite as profuse as that of the tomato and can be scarcely overdrawn. The deeper portion of the taproot is rather poorly branched. But the deeply descending laterals are so well branched and so well furnished with delicate rootlets that it presents an enormous absorbing surface. In contrast to the tomato, the root system of the eggplant is not so widely spreading but penetrates much deeper.
Other Investigations on Eggplant.--Investigations in Germany have shown that the general root habit of eggplant is very similar to that of the tomato. No essential differences were observed either as to lateral spread or branching. The greater mass of the roots attained a depth of about 33 inches. 89
The roots of the eggplant of the Black Pekin variety were washed from the soil at Geneva, N. Y., late in September.
The main roots radiated from the base of the stem at varying angles but the majority rather inclined to the perpendicular. The horizontal roots were smaller in size than those that grew downward and none reached a greater length than 2 feet. Branches were most numerous near the base of the stem. 44
The main roots were 4 to 6 millimeters in diameter.
Many of the cultural practices discussed under tomato also apply to the eggplant.