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The carrot (Daucus carota sativa) is either an annual or biennial plant. Early varieties seed the same growing season in which they are planted. Later ones produce only the whorl or large cluster of fern-like leaves the first year and extend the rough, branching flower stalks to a height of 2 to 3 feet during the second season of growth. This fairly hardy, rather slowly maturing vegetable is common in most home gardens as well as in many market gardens. The "carrot" mostly consists of enlarged taproot but the upper portion develops from the hypocotyl and is a part of the stem.
Seed of the Chantenay variety of carrot was planted Apr. 24 in drill rows 18 inches apart. The seedlings were thinned to a distance of 5 inches in the row.
Early Development.--The first examination of root development was made June 10. The tops were 4 inches tall and consisted of five leaves each, the larger ones had blades 2 by 2 inches in outline and the average transpiring area, as determined by the aid of a planimeter, was only 12.5 square inches.
The plants were characterized by strong taproots 4 to 6 millimeters in diameter. These soon tapered to 1 millimeter in thickness, a diameter held throughout their vertically downward course. Depths of 27 to 32 inches were attained. Compared with other garden plants, the taproots were poorly branched (Fig. 61). No branches occurred in the surface inch of soil but to a depth of 6 inches they arose at the rate of five to six per inch In the deeper soil branches were fewer and unevenly distributed. They varied in number from one to nine (average, three) per inch. Nearly all ran in a rather horizontal direction. They varied in length from 0.2 to 9 inches, the longest Qnes always occurring on the oldest portion of the taproot. Secondary branches, even on the oldest laterals, were not abundant. The last 6 to 9 inches of the taproot were entirely free of laterals. The poor branching and consequently relatively small absorbing area may be correlated with the small transpiring surface afforded by the deeply incised leaves. Even the root-hair development was much less pronounced than on many other garden plants. The roots are very brittle.
Fig. 61.--Taproot system of a Chantenay carrot 47 days old.
Midsummer Growth.--A month later, July 12, the plants had reached a height of 13 inches and attained a spread of 16 inches. The shoot had a thickness of ¾ inch near the base. Each plant had about 14 leaves. Many of these had blades 5 to 6 inches wide and 12 to 14 inches long. Thus the transpiring and photosynthetic area had greatly increased.
The fleshy portion of the taproot extended to a depth of 6 to 8 inches. It had a maximum diameter of slightly more than 1 inch and tapered gradually at 8 inches depth to a thickness of about 2 millimeters. In the deeper soil (below 18 inches) the taproot was only 1 millimeter thick. Its course through the deeper soil was quite tortuous. Abrupt turns and deviations of 0.5 to 2 inches from the vertical were frequent (Fig. 62). Depths of 4 to 4.8 feet were usual.
Fig. 62.--Root system of a carrot a month older than that shown in Fig. 61.
Only a few, short branches arose in the surface 2 inches of soil. These were unbranched. Between 2 and 9 inches in depth, however, 45 to 55 laterals originated. Usually 16 to 24 of these spread horizontally for distances of 15 to 24 inches. A few of those which originated deepest (5 to 9 inches) turned downward and frequently extended to the 2- to 2.7-foot level. Although the other branches in this region of the taproot were shorter, thread-like, and poorly branched, these longer roots (about 1 millimeter in diameter) were clothed with laterals at the rate of 5 to 9, but sometimes 10 to 15, per inch. They were 4 millimeters to 2 inches (rarely more) in length and most of them were unbranched. A few of the main laterals that had been injured terminated in brush-like mats of rootlets. Below the 9-inch level roots arose at the rate of three to four per inch throughout the course of the taproot except on the last 6 to 9 inches which were devoid of branches. Below 3.5 feet these were short and unbranched. Nearly all pursued a rather horizontal course.Many short, unbranched or meagerly branched roots alternated with a few of greater length which were very well clothed with rootlets. The relative lengths, degree of branching, etc. are clearly shown in Fig. 62 where a carefully selected root is pictured. The older roots vary in color from tan to yellow. Only the younger portions are white.
Maturing Plants.--At the final examination, Aug. 12, plants of average size had 16 fully grown leaves and 4 to 6 partly developed ones. Some of the older leaves had considerably deteriorated. The fleshy portion of the taproot was about 8 inches long and 1.5 inches in diameter near the soil surface but less than ½ inch thick at the 8-inch level, the Chantenay being a half-long, stump-rooted variety. The average number of branches at the several depths is shown in Table 15.
TABLE 15.--NUMBER OF LATERALS ORIGINATING FROM THE TAPROOT OF CARROT Depth Small Large Depth, Small Large inches branches branches inches branches branches 0-1 7 0 14-15 5 0 1-2 9 1 15-16 4 0 2-3 13 0 16-17 7 0 3-4 5 0 17-18 7 1 4-5 6 2 18-19 4 1 5-6 5 3 19-20 2 0 6-7 7 1 20-21 6 3 7-8 5 0 21-22 11 0 8-9 4 0 22-23 10 1 9-10 3 1 23-24 6 1 10-11 1 1 24-25 6 2 11-12 1 1 25-26 3 0 12-13 11 0 26-27 7 1 13-14 7 1
Thus on the first 2.3 feet of a taproot of average size approximately 20 large roots and 160 small ones had their origin. The large ones varied from 1 to over 2 millimeters in diameter. A few near the surface ran outward 12 to 18 inches and, like the network of shorter, smaller roots originating from the fleshy portion of the carrot, branched profusely and ended in the surface soil. But many of the larger roots, after pursuing a horizontal course for 1 to 2 feet (maximum spread, 2.3 feet) turned downward. They then ran almost straight downward ending in the 3- to 6-foot level. A comparison of Figs. 62 and 63 shows the characteristic development of these roots. At the earlier stage of growth (July 12) they had not reached their maximum spread. Moreover, only a few were utilizing the moisture and nutrients of the deeper soil.
Secondary branches on all of the roots were not only more abundant (4 to 13 per inch) but also longer (now 2 to 3 inches) than at the previous examination. Considerable variation occurred in the distribution of laterals. Some were 6 inches in length. Branches of the third order were nowhere abundant. An examination of the drawing shows that many new roots had arisen from the deeper portions of the taproot where they averaged 7 per inch. The average length, however, was only slightly greater than at the previous examination. The taproots were traced to their ends at depths of 6 to 7.5 feet. The roots were still growing vigorously. Judging from the development of such crops as parsnips (see Chapter XXIV) and beets (see Chapter VIII) excavated in September, it seems certain that the roots of the carrot reach a working level of 6 to 7 feet and a maximum depth at least of 10 feet.
Fig. 63.--Root system of carrot on Aug. 12. The roots were still growing vigorously.
Growth of Carrots during Winter and Spring.--Well-developed carrots of the Long Orange variety, with the fleshy taproots 2 inches in diameter, were left in the soil at Norman, Okla., at the end of the 1925 growing season. Because of late-summer drought there were only a few green leaves during August and September. Death of all or nearly all of the absorbing roots in the surface foot of soil resulted from the drought. Practically no growth either above or below ground occurred from the middle of August until after the September rains. By Dec. 3 there was a vigorous growth of leaves to a height of 8 inches and the roots also had made renewed growth. From the same areas on four sides of the fleshy taproot that had previously given rise to laterals, clusters of new rootlets occurred. These originated in clumps from the enlarged, bulging, meristematic areas, usually 5 to 25 occurring in a single cluster. From 500 to 1,500 were counted on individual plants. The roots were hairlike, a few millimeters to 7 inches in length and invariably pursued a horizontal course in the mellow, moist soil. Thus the older surface-absorbing system, only fragments of which remained intact, was being replaced by this newer one. The deeper and younger portions of the old root system were apparently functioning in a normal manner.
Further studies were made at the end of February. The plants had intermittently grown during the warmer winter weather, since soil moisture was very favorable (p. 19). The new roots, formed in the fall, were now 8 to 16 inches long. They had not deviated from their horizontal course. The larger ones were well furnished with numerous short laterals which greatly increased their absorbing area. Moreover, new laterals were developing on the old taproot to a depth of 2 feet as well as on the larger laterals near their place of origin.
On Apr. 16, when it became evident that the rapidly growing tops would need considerable room, the plants were thinned to 2 feet apart in the row.
Mature Plants.--By June 22 the plants had reached their maximum development. The numerous leafy stalks, 15 to 40 in number and 8 to 16 millimeters in diameter, reached heights of 30 to 42 inches. They were just ready to blossom. Roots were exceedingly abundant in the surface foot of soil; on one plant there were 109 with a diameter of 1 to 2 millimeters and 430 finer ones in this soil layer. A maximum lateral spread of 30 inches was found. For example, one strong lateral, originating at a depth of 4 inches, ran obliquely outward and downward to a depth of 29 inches at a horizontal distance of 27 inches from the plant. Here it turned downward and ended 1 foot deeper. This illustrates the marked development of some of the new roots. Most of them, however, were shallower and shorter. The horizontal course of those arising within 2 to 3 inches of the surface and the more oblique direction of growth of those originating somewhat deeper were very characteristic. With its dense network of branches, this new root system rather thoroughly occupied the surface 12 inches as well as portions of the deeper soil. It greatly supplemented the activities of the deeper portion of the old root system.
The taproots reached a maximum depth of 55 inches. Strong, much-branched laterals occurred mostly in the second foot of soil. These spread 12 to 18 inches from the taproot and then often turned downward. Throughout its course below the first foot short laterals arose from the taproot at the rate of 4 to 12 per inch.
Summary.--Carrots are characterized by a strong, deep, well-developed taproot system. Plants with tops in the fifth-leaf stage have taproots 2.5 feet deep. Branching throughout is very poor. The greatest branching is in the surface 2 to 4 inches of soil where a few laterals extend horizontally 8 to 10 inches. During the following month the taproot grows 2 feet deeper. A fairly profuse network of horizontal branches extends from the lower half of the fleshy portion of the taproot 1.5 to 2 feet outward into the surface soil. A few major branches descend rather vertically, supplementing the absorbing area of the now better-branched taproot, to a depth of nearly 3 feet. About the middle of August, maturing, plants have well-formed "carrots" from which fine roots arise in great abundance. These furnish an excellent surface-absorbing system near the plant. Many of the formerly horizontal laterals have turned vertically downward after reaching a maximum spread of 2.3 feet. They give rise to small laterals only but extend through the third and fourth and often the fifth foot of soil. Well-branched taproots reach the 7.5-foot level. The roots are still growing vigorously and undoubtedly extend much deeper.
When roots die from drought, they are replaced by multitudes of new ones of a very fibrous nature but often 1.5 to 2.5 feet in length. A dry surface soil tends to promote a vigorous development of strong laterals from the deeper portions of the taproot.
Other Investigations on Carrots.--Carrots of the French Forcing and Long Red Altrincham varieties were examined in the middle of September at Geneva, N. Y.
On both the taproot was small and soon tapered into a filament. We traced it downward 16 inches, at which depth it was too delicate to follow further. The horizontal roots apparently extended little more than 1 foot. The fibrous roots chiefly proceeded from the taproot, though a few started from near the base of the thickened part. These extended both deeply and shallowly, some rising nearly or quite to the surface, while others apparently penetrated as deeply as did the taproot. 43
It seems clear in this case that the fineness of the root was an obstacle to its complete recovery by the method employed of washing away the soil.
Carrots of the Long Orange variety were set out in the spring at the same station and examined late in September of the second year of growth. The leading roots were found to extend quite as far as those in plants of the first year's growth. The fibrous roots, however, appeared less numerous. No branches from the horizontal roots extended upward to near the soil surface as was the case in both of the varieties grown for a single season. The lower roots were developed to a greater extent than in those plants grown directly from seed. 43
At Saratov in southeastern Russia, the roots of the carrot have been traced, to a depth of 40 to 52 inches. Branching was very abundant in the plowed soil layer and the plant thus well adapted to absorb the water afforded by the summer precipitation. At greater depths the branches were larger and occupied the soil rather thoroughly, being especially well developed where they occupied old burrows and earthworm holes. 76a
Investigators in Germany state that the roots of carrots are characterized by their great depth of penetration which was found to be about 5 feet. The root system was much like that of the lettuce (see Chapter XXXV) in so far as the separate root branches penetrated rather vertically downward and did not spread widely. But they differed in that adventitious roots were lacking in the carrot and branches of the first order furnished the means of lateral spread. These laterals were found to be rebranched in a manner similar to the lettuce. 89
Experiments in the vegetable gardens at Ithaca, N. Y., where carrots were grown in a gravelly sandy loam are of interest . The plants were grown in rows 18 inches apart. The taproot and several other larger roots arising from the side of the carrot reached depths of 30 inches. These roots produced almost countless numbers of branches which were themselves rebranched at least to the second order. The soil directly beneath the plant was filled with roots to a depth of 25 to 30 inches. A space 4 to 6 inches wide in the center between the rows was not so completely filled although at a depth of 4 to 8 inches many roots met and overlapped between the rows. 152, 159a
Root Habit in Relation to Cultural Practice.--The amount of soil that must be pushed aside and the extensive development of roots in the surface soil helps to make clear the benefits of a deep, loose, mellow soil. Such a soil is also beneficial in preventing the formation of a soil crust over the slowly germinating seeds and about the slowly developing plants. The carrot has a delicate root system during its early stages of growth and one poorly adapted to penetrate stiff, hard soil. Muck, a fine, loose-textured soil, is an ideal one for the growth of carrots and indeed for most root crops. Since the plants grow slowly, they cannot successfully compete with weeds; hence the value of clean and thorough but shallow cultivation. It has been shown at Ithaca, N. Y., using both early and late carrot crops, that little or no benefit is derived from cultivation other than preventing weed growth. Owing to the shading of the soil by the tops and the consequent lowering of soil surface evaporation, together with the thorough occupancy of the shallow soil by a dense root growth, soil moisture was not conserved by cultivation. Plats where the weeds were kept out by scraping the surface usually had slightly less moisture than those repeatedly cultivated, but in some cases these conditions were reversed. 152,158,159a
For the best growth of the plant the usual spacing, 4 to 8 inches apart in rows 12 to 18 inches distant, is too close. Competition between closely spaced plants results in considerable dwarfing. When vegetable crops are crowded into a small area, garden practices, such as furnishing the plants with unusually large supplies of nutrients, conserving the water supply, and eliminating weeds, are employed to stimulate growth. Competition, however, is always severest where plants of the same kind are grown in close proximity. All make the same demands for water, nutrients, and light at the same levels and at the same time. The vegetable grower is concerned, however, with the best interests of the plant only in so far as they meet his needs. By growing carrots thickly in favorably rich, moist, loose soil, he prevents the plants from attaining too great a size and secures an abundance of well-formed, succulent, medium-sized roots.