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The radish (Raphanus sativus) is one of the commonest of garden vegetables, being especially in favor with the home gardener because of the ease and rapidity with which an early spring crop may be obtained. If planted early, it forms seed and completes its life cycle in a single season. Varieties planted later set seed during the second season of growth. The branched flower stalks are 1 to 3 feet in height. The plants are grown for their pungently flavored roots, including the upper portion of this swollen, edible part which is morphologically a stem. The "roots" vary much in size and shape from those that are short and globular, through conical and oblong, to the spindle-shaped type.
Seed of the Early Scarlet Turnip White-tipped radish was planted Apr. 10. The rows were 14 inches distant and the plants thinned to 2 inches and later to 4 inches apart in the row.
Early Development.--Radish seeds germinate and produce a taproot system in a remarkably short time. They have been observed to form six to eight laterals on the upper inch of the taproot only 4 days after planting and at a time when the cotyledons were just unfolding. Nine days later, when the plumule was appearing, secondary laterals had been formed. This shows that a plant needs an efficient absorbing system before much leaf area is exposed to the dry air.
On May 18 the plants each had about five leaves and other leaves were just appearing. The average leaf surface was 22 square inches. The total spread of tops was about 6 inches.
The fleshy portion of the strong taproot was 1 to 1.5 inches in diameter. The taproot pursued a nearly vertically downward course, curving laterally through short distances. An average depth of 19 inches and a maximum penetration of 22 inches were reached. The diameter of 2.5 millimeters, found just beneath the enlarged portion of the taproot, decreased at a depth of 6 inches to only 1 millimeter, a thickness that was maintained throughout the course of the root. Just below the enlarged part of the taproot, or at most only a few millimeters below, numerous branches arose (Fig. 46). These extended outward in a generally horizontal direction but often curved upward or downward. The longest reached a distance of 12 inches from the base of the plant, but most of the roots were only 2.5 to 3 inches in length.
Fig. 46.--Root system of the Early Scarlet Turnip White-tipped radish about 5 weeks old.
Laterals were thickest on the first 3 inches of the taproot, where they occurred often in groups of twos or threes, in two more or less distinct rows on opposite sides of the taproot. From 40 to 45 roots often sprang from the first 3 inches of the slender portion of the taproot. Below this they were fewer, shorter, and more poorly branched or entirely destitute of laterals. They, spread horizontally on all sides of the plants. Branching was very profuse. Laterals of the first order occurred at the rate of 5 to 20 per inch and varied from 1 millimeter to 2 inches in length.
Although branches of the second order were found but rarely, the thread-like roots of these young plants thoroughly ramified the surface layer of soil.
Effect of Soil Structure on Root Development.--Radishes were grown in rectangular containers with a capacity of 2 cubic feet and a cross-sectional area of 1 square foot. A rich, sandy loam soil of optimum water content was screened and thus well aerated. One container was filled with soil with very slight compacting. It held 173 pounds. Into the second container, 232 pounds of the soil were compacted. Surface evaporation was reduced by means of a thin sand mulch. When the plants were 3 weeks old and five or six leaves had developed, the side of each container was cut away and the root system examined. The taproots had reached a maximum depth of 22 inches in the loose but only 5 to 8 inches in the compact soil. The laterals in the first 2 inches of dense soil were more numerous, of somewhat greater diameter, and slightly longer (maximum 6.5 inches). They were also much more branched. But in the loose soil branching occurred throughout to near the root tip. That roots of many species penetrate more deeply in loose than in compact soils and that under the latter condition branching is increased has been shown by numerous investigations. 87, 163, 48, 69, 2
Two-months-old Plants.--A second examination was made June 11. The plants now had stems nearly 2 feet tall. with 9 to 11 branches. They were flowering abundantly. A few of the oldest leaves had turned yellow and dried. The cauline leaves were relatively smaller than the basal ones. Plants of average size had leaf surfaces slightly less than 3 square feet in extent.
At this time the fleshy portion of the root was about 2.5 inches thick. On about half the plants a second storage reservoir was being formed below the first, or at least the root was considerably thickened. The strong taproot, about 4 millimeters thick throughout the first 8 inches, which was the most branched part of its course, ran quite vertically downward to a depth of 3 feet. The root ends were not thick as at the preceding examination but greatly attenuated and quite free from branches for distances of 5 to 7 inches. In fact below 10 inches branching was relatively sparse although greatly promoted where the roots entered earthworm burrows or soil areas enriched by the presence of former roots. Here they sometimes coiled about and branched much more profusely (Fig. 47).
Fig. 47.--Characteristic root habit of a 2-months-old radish. Most of the fibrous roots are distributed in the surface soil.
A remarkable degree of branching occurred between the depths of 2 and 8 inches. In one representative plant 49 laterals arose from the first inch of taproot below the enlarged part, 34 from the second inch, 20 from the third, and 11 to 15 from each of the next 3 inches, respectively. Below this, in the first foot, the taproot branched at the rate of 5 to 7 laterals per inch. Many of the branches arose in groups of twos or threes from two rows on opposite sides of the root. They usually extended horizontally in all directions from the taproot. Many were only 2 to 4 inches long; some reached distances of 13 to 17 inches from the base of the plant. That they, like the taproot, were growing rapidly was clearly indicated by the long, unbranched root ends. The laterals were well furnished with branches which occurred at the rate of 4 to 10 per inch. Although many of these were very short (0.2 inch or less), others reached a length of 1 to 3 inches. Most of the older ones were rebranched with short laterals and branches of the third order were frequent. The thread-like character of the branches and the great network of delicate branchlets are very characteristic of the radish and make it quite difficult to excavate.
Mature Plants.--A final examination was made July 13 when the plants were 26 inches tall and in the flowering stage. The vigorous plants had 18 to 22 leaves, usually with a flower stalk in the axil of each. The large transpiring surface (nearly 5 square feet) may be visualized when it is recalled that the larger leaves were 8 to 12 inches long and 4 to 6 inches in width.
Just below the soil surface the enlarged globular portions of the fleshy taproots were 2.5 to 4 inches in diameter. Although usually cracked and fissured, they were still quite firm and well stored with food. Below this portion secondary, irregular enlargements occurred on practically all of the many plants examined. These varied from 0.5 inch to over 2 inches in diameter, the taproot frequently being swollen for a distance of 6 to 8 inches (Fig. 48).
Fig. 48.--Root system of radish that had formed a flower stalk, excavated July 13. The insert (A) is a continuation of the taproot which reached a depth of more than 7 feet.
Even a casual inspection of Fig. 48 shows the marked growth the roots had made since the examination a month earlier. The taproots frequently reached depths of 6 to 7 feet, and a maximum penetration of 7.2 feet was found. The widely spreading, horizontal lateral branches had extended the absorbing area to over 3 feet on all sides of the plant. A maximum lateral spread of 4.1 feet was ascertained. Not only was the surface soil from 3 to 12 inches thoroughly ramified over a wide area but also the deeper soil, even beyond 4 feet, was penetrated by strong, well-branched lateral roots.
The taproots pursued a generally vertically downward path but with characteristic kinks and curves as indicated in the drawing. Throughout their course in the surface 4 to 12 inches of soil they were so covered with branches, both large and small, that they formed a cobwebby network. Below 12 inches, the taproots, now about 2 millimeters in diameter, were very much kinked and curved and sometimes turned abruptly. Large laterals were not abundant, frequently only 5 to 8 occurring below the first foot of soil. These were rather poorly furnished with short branches and seldom exceeded 8 inches in length. Their direction of growth was mostly horizontal. No branches over 1 to 2 inches long occurred at greater depths, but numerous, short branches were found throughout the course of the taproot.
The larger lateral roots, originating in the surface 4 to 12 inches of soil, usually occurred at the rate of six or more per plant. They were 1.5 to 2 millimeters in diameter throughout the first 8 to 12 inches of their course which, as among the very numerous smaller laterals, was characterized by many kinks and turns. Frequently, the main laterals gave rise to branches quite as large as themselves. All branched profusely, frequently to the third and fourth order, especially the older parts. Portions of the laterals were found that had only three to four branches per inch or, indeed, were destitute of them; they were typically clothed with rootlets 0.4 to 1 inch long at the rate of 8 to 12 per inch.
As a whole the root system of the radish is rather clearly differentiated into a deeply penetrating portion consisting of the taproot and a few of its widely spreading and deeply penetrating branches and a shallower part which spreads widely but is almost confined to the surface 4 to 12 inches of soil. Here dense masses of delicate, cobwebby roots were profuse. This soil layer was so thoroughly exhausted of its water that during drought many of the cobwebby rootlets of the intricately branched network of roots shriveled and died. The absence of roots in the surface few inches of soil is characteristic.
Summary.--The Early Scarlet Turnip White-tipped radish is characterized by a rapidly growing and deeply penetrating taproot. At the time of. the removal of the fleshy portion of the root for table use, the root system is not extensive when compared with that of most vegetables. Although it may penetrate to a depth of 2 to 3 feet and have a lateral spread of 12 to 16 inches, most of the absorbing area lies in the surface 2 to 8 inches of soil. Even on fully matured plants only the portion of the taproot lying in the 2 to 12 inches of surface soil gives rise to large laterals. Most of these extend horizontally to distances of 3 feet or more on all sides of the plant. They branch profusely and rather thoroughly fill the surface soil with a network of absorbing rootlets. The deeper soil is ramified by the taproot and by a few of the major roots or their branches which originate in the surface foot. The taproot penetrates to depths of 6 to 7 feet. The large branches reach depths of 3 to 4 feet and are well clothed with short laterals.
Seed of the Early Long Scarlet radish was planted at Norman, Okla., Apr. 18. Conditions for growth were very favorable both as regards temperature and soil moisture and the plants grew vigorously. When 20 days old and at a time when the tops consisted of five to seven leaves, the first root examination was made.
Early Development.--The taproots, some of which were already 0.5 inch in diameter, had grown at the rate of 1.2 inches per day. They reached a maximum depth of 2 feet. Branches were found from just beneath the soil surface to near the root ends. They were very abundant, 37 to 53 occurring on the first foot of the taproot alone. In length, they ranged from a few millimeters to 15 inches. Their direction of growth was mostly horizontal although below 8 inches a few ran obliquely outward and downward. All of the longer ones were branched. Branches occurred at the rate of about 4 per inch and many of them were 1 to 2 inches long. These also were rebranched. The entire root system was covered with root hairs and was apparently functioning vigorously. Most of the absorbing surface occurred in the first foot of soil, some of the laterals extending almost to the soil surface. But the second foot was also ramified by the taproot, by a few obliquely descending major laterals, and by the rather numerous branches from these.. The second foot of soil was moist and fairly mellow and was soon to become the seat of vigorous root activity. At this time the root system had, in general, the shape of an inverted cone 2 feet long and about 28 inches wide.
Later Development.--Twelve days later, May 20, the fleshy portion of the taproot had increased to 1 inch in diameter. The roots were enlarged to a depth of about 8 inches. Some of the major branches were also fleshy near their origin. Marked changes had occurred during the 12-day interval. The depth of penetration had increased to 3 feet and the lateral spread to a maximum of 2 feet. Although the general shape of the root system had not changed, many of the secondary laterals had greatly elongated; the absorbing area was much extended; the second foot was well ramified to a distance of 8 inches on all sides of the taproot, and the third foot was almost as fully occupied as was the second 12 days earlier. Branching throughout was even more abundant. The root system was growing vigorously.
Mature Plants.--The plants continued their rapid growth throughout June and by July 6 they averaged 30 inches in height. The tops spread so widely that they nearly covered the soil, although they had been thinned earlier until the mature plants were 18 to 24 inches apart in rows 3.5 feet distant. Blossoming had ceased and the seed pods were maturing.
The fleshy roots were 3 to 5 inches in diameter and 8 to 14 inches long. The taproots were several millimeters in diameter, however, even to a depth of 3 feet. They pursued a generally downward, although somewhat tortuous, course through the rather compact subsoil and reached depths of 4.5 to 5 feet. Several of the major laterals, originating from the fleshy portion of the root or near its base at depths of 8 to 10 inches, had a lateral spread of 40 inches. Many of these ran far outward and then turning downward penetrated to depths of 2 to 3 feet. Others pursued a horizontal course throughout but gave rise to long, vertically descending branches some of which reached the 4-foot level. Lateral branches on all of the roots were very much longer than formerly, penetrating the soil in all directions for distances of 6 to over 30 inches.
In the process of growth expansion the plants had forced themselves 1 or 2 inches out of the soil. Similar root heaving has been observed on other fleshy rooted plants. 83 This was clearly shown not only by the exposed crown and upper portion of the fleshy root but also by the abrupt inward curving of the main roots originating from the fleshy part of the taproot. Earlier examinations showed this same type of root running horizontally. The phenomenon was very pronounced since numerous curved branches 2 to 10 millimeters thick all turned obliquely downward at their origin. Some of the shallower horizontal roots were actually stripped of their laterals and pulled above the soil surface.
Owing to the very dry soil and perhaps in part to high temperatures, many of the roots in the shallower soil had ceased absorbing; others had died. Very few active roots occurred within a radius of 2 feet from the base of the plant and to a depth of over 12 inches. Thus absorption was confined to the younger parts of the horizontal branches and to those lying deeper in the soil. The root system was very extensive, however, both in lateral spread and depth thus ramifying a very large soil volume. Throughout this soil mass, except as already indicated, branching was very profuse; great mats of glistening white rootlets filled the soil. It is remarkable that a plant in such a short period of growth can produce such an extensive and intricate absorbing system.
Summary.--The root system of the Early Long Scarlet radish is of an entirely different type from that of the Early Scarlet Turnip White-tipped variety. In its early development the vertically penetrating taproot was branched throughout, mostly with horizontal laterals, from the soil surface to near its tip, the root system having the general shape of an inverted cone. When the roots have reached the size used for eating, they penetrate to depths of 3 feet and the much-branched laterals spread radially to distances of 2 feet, the conical nature of the root system being in general retained. Mature plants are characterized by very fleshy taproots, 4.5 to 5 feet long and many major laterals, all of which arise in the surface foot of soil. These spread widely (maximum, 40 inches) and often turn downward near their ends. They also give rise to major laterals which penetrate deeply. Thus, although the surface soil is well occupied, the subsoil to depths of 3 to 4 feet is also more or less filled with the much-branched lateral rootlets. Further investigation may show that these differences in root habit are due in part to varietal characters and in part to soil environment.
Other Investigations on Radish.--The root systems of the Gray Summer Turnip variety and the London Particular Long Scarlet were washed from the soil at Geneva, N. Y.
The roots of both penetrated the soil a distance of 2 feet and the branches extended on either side more than 21 inches, mingling with those from adjoining rows. The taproot did not begin to branch much until some distance below the edible part. The branches at first were few in number, usually but two or three from the taproot. These extended nearly horizontally, and ramified toward their extremities into many fibrous roots. The greater part of the feeding roots lay in the upper 8 inches of the soil. Though the edible roots of these two varieties are quite different in f orm, their rooting habits show no difference. 43
Certain German investigators have also observed that the fleshy portion of the main root is practically without branches; that the taproots reach depths of 12 to 20 inches; and that the ultimate branching is very diffuse, branches of the third order being abundant. 89
Steaming the soil has been found to increase the total number, size, and yield of radishes, a fact which is undoubtedly connected with a greater development of the root system (cf. Chapter XXVI).139
Root Development in Relation to Cultural Practice.--Although the radish, like its wild ancestors, will grow in nearly all types of soil, a light, friable) fertile soil is best. Since the crop develops very rapidly and the root system, except in plants grown for seed, is not extensive, the most favorable conditions for root activity should be attained by thorough seed-bed preparation and shallow cultivation. Stirring the 2 inches of surface soil will not harm the roots, since practically all of them lie deeper. They do nearly all of their absorbing in the surface 6 to 8 inches. Hence, water should be conserved in this soil layer, manure and fertilizers supplied to it abundantly, and competing weeds kept out so that growth will be rapid and continuous.
It is well known that radishes will thrive under close planting or as an intercrop between rows of later-maturing plants. A knowledge of the shallow, non-extensive, but well-branched root system helps one to understand why this is the case. The ramification of the soil by the roots of older plants and probably by longer-season varieties is quite a different matter. Undoubtedly close planting, which results in shading aboveground as well as root competition in the soil, affects the size and development of the fibrous portion of the root system just as it does the edible part. The effect of shading on the growth of roots has been clearly demonstrated, for example, in the case of certain tree seedlings.
In white-pine seedlings grown in Vermont
. . . darkness induces the growth of tall seedlings with poorly developed roots; a diffuse light, the growth of shorter plants and longer roots; and the full light produces short stocky plants with long branching roots. 20
For example, seedlings of similar age grown in the nursery under full shade had unbranched taproots 3.5 centimeters long, those grown in half shade were 4.5 centimeters long and had the beginnings of laterals, And seedlings grown in full light had taproots 5.2 centimeters long and a lateral development of roots nearly seven times as great as those in half shade. Among 50 seedlings excavated about 3 weeks later, the length of the root systems were 4.5, 8.2, and 13.8 centimeters and the total number of lateral branches on the lot 5, 143, and 468, respectively.
Plants that are crowded cannot develop efficient root systems and their activities are confined to the shallower soil. 173 Unless this is kept very rich and well moist, drought and reduced yields are inevitable. Since plants with like root systems are making similar demands at the same depth at the same time, they are least fitted to be crowded together. Intercropping and growing mixed cultures more nearly approaches nature's method of producing a dense growth of vegetation. For scientific progress in this direction a thorough understanding of root relations is indispensable.