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Leek (Allium porrum) is a very robust, biennial plant. Bulbs are nearly always absent, the plant being grown for its blanched stems and leaves. The plants are propagated entirely from seed which is sometimes sown in the greenhouse or hotbed and the seedlings later transplanted into the field.
Early Development.--Leek of the Large London variety was planted at Norman, Okla., Mar. 16. The seeds were thickly sown to insure a stand, and later the plantlets were thinned to 4 inches distant in the rows which were 3.5 feet apart. Weather conditions were not favorable for germination or growth, and the plants developed slowly. When they were nearly 2 months old (May 19) and had reached a height of 10 inches, the first root examination was made. Each plant had four leaves approximately 1/2 inch in width.
An average of 16 roots, in all stages of development, was determined per plant. As a rule, three were less than 1 inch long; four were 6 inches or less in length; the remainder grew to lengths of 7 to 14 inches. Their direction of growth was very similar to that of the onion, although, in general, they spread to a greater extent (Fig. 11). A maximum lateral spread of 8 inches and a depth of 14 inches were found. The white, tender roots were free from abrupt turns and short curves but pursued a rather wavy course through the mellow soil. They were 1 millimeter thick and the older ones had given rise to short laterals at the rate of about two per inch.
Fig. 11.--Roots of a two-months-old plant of leek.
Half-grown Plants.--From this time the plants grew rapidly, owing to a favorable supply of soil moisture. By the middle of June they were 15 inches tall, possessed bulbs 0.5 to 0.8 inch thick and leaves 1 inch in width. The roots had increased in number to an average of 38 per plant. Most of them ran 6 to 8 inches more or less horizontally outward and then downward; the others ran obliquely or, frequently, nearly vertically downward (Fig, 12). A maximum spread of 14 inches was attained at a depth of 6 to 8 inches and some of the vertical roots reached a depth of 20 inches. In penetrating the more compact subsoil, the roots often pursued a tortuous course and were more kinked than in the first 8 inches of mellow soil.
Fig. 12.--One-half of the root system of leek, 3 months old.
Maturing Plants.--The leek plants were in fine condition and still growing vigorously on July 26 at the final examination. Each had about a dozen leaves, the largest being 2 to 2.5 feet long and 1.5 inches wide at the base.
Fig. 13.--Leek late in July of the first season's growth. The root system was still developing vigorously and would probably have extended much deeper. Only about one-fourth of the roots are shown.
A typical plant had 119 fibrous roots extending in all directions from the base of the stalk from horizontally to vertically downward. Many of these roots were only 1 millimeter in diameter but the larger ones were 1.5 millimeters thick, at least throughout the first foot of their course. The general direction of growth may be seen at a glance (Fig. 13) where it may be noted that the lateral spread had been increased to 21 inches and the maximum depth to 30 inches. A comparison of Figs. 12 and 13 shows that not only is the surface foot very much more thoroughly ramified by the root network but also that the second foot of soil is thoroughly occupied. Indeed, the 12- to 24-inch level was now the seat of great activity and it seems almost certain that in later stages of development similar root activity might be found in the deeper soil.
Tiny laterals usually began to appear about 2 inches from the root tips. These increased in length on the older portions of the root, reaching a maximum of 2 inches a foot from the root ends. On some of the roots, branches occurred quite irregularly. Thus considerable portions of the roots were free from laterals. On others an almost uniform distribution of two rootlets per inch was found throughout their length. On the larger roots, within a radius of approximately 12 inches from the base of the plant, larger branches occurred. Frequently, a length of 10 inches was attained by these laterals, which were, however, only rarely rebranched. The number of laterals per inch of main root had not increased but they were longer than before and occurred over a much larger root area.
Summary.--Leek has a fibrous root system which consists of 50 to 100 or more main roots. Many of these spread 14 to 21 inches just beneath the soil surface where they may end or, more usually, turn downward. Others pursue an obliquely downward course so that the soil is ramified with roots of mature plants to a depth of 18 to 24 inches. The branches are moderately few with a maximum length of 10 inches but they rarely rebranch.
Compared with the onion of equal age the root system of leek spreads more widely, a fact that may be correlated with the practice of spacing leek plants a little farther apart than onions. Otherwise cultural practices are similar. The top development of leek is also considerably greater than that of the onion. The depth of penetration is much less but this is partly compensated by the much longer laterals and the more thorough distribution of roots in the surface soil. More profuse branching and more thorough occupancy of a wider area of soil have also been found by other investigators. 89 In both species the laterals are only rarely rebranched. To what degree these differences are heritable and to what extent they may be modified by environment await experimental investigation.