Beans belong to a family of plants (the Leguminosae) which, with the possible exception of the grasses, ranks first in agricultural importance. There are many genera, species, and varieties of beans. The most important beans grown in the United States are the kidney bean (Phaseolus vulgaris) and lima beans (Phaseolus limensis and P. lunatus).
Common or Kidney Bean
The kidney bean is an annual, tall-twining plant. Dwarf varieties or bush beans (P. vulgaris humilis) are of low stature and of non-climbing habit. It is by far the most important species of bean grown in the United States. It is found in nearly every vegetable garden and is grown extensively on a commercial basis. There are 150 varieties of kidney beans in America. 66
The Wardwell's Kidney Wax variety was used for root investigations. This is a wax-podded variety of bush bean. The crop was planted May 18, an earlier planting having frozen. The plants were spaced 6 inches apart in rows 2.5 feet distant.
The bean seedling is characterized by a very vigorous development of the taproot. Plants only in the cotyledon stage, but grown in warm, mellow soil, often have well-branched taproots 12 inches long.
Root development in the field was first examined June 18. The plants were already 5 to 6 inches tall and had a spread of 10 inches. Each had about 10 leaves, the leaflets being 3 to 4 inches long and 2 to 3 inches broad. The total leaf surface averaged 1.4 square feet.
The root system was characterized by a strong, nearly vertically descending taproot and multitudes of long, horizontal branches arising mostly in the 2- to 6-inch soil level (Fig. 53). Some of these large branches, as in the lima bean, were adventitious, arising from the base of the stem.
As a rule many of the shorter, horizontal roots, only 2 to 4 inches long, were unbranched or only poorly furnished with laterals. On the longer ones branching occurred at the average rate of 10 branchlets per inch. These varied from 0.2 to 2 inches in length. A very few were 4 to 8 inches long and possessed branches of the third order. Thus the soil to a depth of 8 inches was quite well occupied by a network of rootlets. These are shown in Fig. 54 which is a surface view of the roots in the first 6 inches of soil.
A comparison of the horizontal view with the vertical one gives an adequate picture of the root extent and position. That the roots were elongating rapidly was shown by the 3 or more inches of unbranched root ends. Also the future, obliquely downward course could already be predicted by the position of the root termini.
The taproot, below the 8-inch level, was well clothed with rootlets (Fig. 53). Only those exceeding 3 to 6 inches in length were rebranched. The last 5 inches of the turgid, thick, white taproots were quite smooth, although considerably kinked and curved from penetrating the compact soil. A maximum depth of 27 inches was ascertained.
A second examination was made about a month later, on July 14. The plants were now nearly 1 foot high and had a spread of approximately 1 foot. Those of average size possessed about 18 large leaves, the largest of which were over 1 foot in width. The largest leaflets had a length and diameter of 6 and 5 inches respectively. The transpiring surface had increased to 5.5 square feet. The plants were blooming profusely and numerous young fruits were growing vigorously.
The roots too had made considerable growth, although this was not so marked as in many other vegetable crops. The lateral spread reached a maximum of 30 inches, the most widely spreading roots sometimes ending in the surface foot of soil.
Where the tips of the roots had been injured or destroyed, numerous laterals arose from the root ends and some of them pursued the course which would have been taken by the main root. This phenomenon of the growth of laterals being promoted by injury to the main branch has been repeatedly observed among many vegetable and field crops.
The taproots had also increased their depth from about 27 inches at the June examination to 3 feet. Likewise the working level (exclusive of taproot) which scarcely exceeded 12 inches, had now been extended to an approximate depth of 20 inches.
As before, considerable variation was found in the total number of large main roots. On the plant selected for drawing, most of these, as in the lima bean, arose from the base of the enlarged stem. Sometimes the taproot furnished more of them.
These strong main or lateral roots had diameters of 2 to 3 millimeters and were at least 1 millimeter thick throughout their course. The rate of branching was, as before, about 10 laterals per inch of root. They varied from 3 to 15 but usually approached the higher number. Many of them were only 0.2 to 1 inch in length, especially on the younger portions of the older roots and on the longer branches of the old ones. The latter were now abundant and ranged from over 1 to 6 inches in length. They were usually profusely rebranched, roots of the fourth order being not uncommon.
As regards the taproot, it plays an important role. Filling the soil with its numerous branches just below the plant and extending considerably beyond the working level of the other roots, it continually drew upon new sources of supplies. Although some taproots were quite vertical in direction of growth, others pursued a very devious route as is shown in the drawing.
A final study of the kidney bean was made Aug. 5. The very leafy plants were about 13 inches tall and had a total spread of 15 inches. Although some were still blossoming, most of them had an abundance of mature pods. That they were fully grown was shown by the drying of some of the leaves.
The roots had not increased in lateral spread but the taproots and some of the longest main laterals (including those arising adventitiously from the stem) had extended to depths of 40 to 46 inches, The working level had also been greatly increased, i.e., from 20 to 36 inches. Root branching was very well developed to the 3-foot level and to a distance of 2 feet on all sides of the plant. Many roots extended deeper. A few, especially the deeper portion of the taproots, showed signs of decay.
The kidney bean rapidly develops a deeply penetrating taproot. On plants 31 days old and 6 inches tall this reaches a length of over 2 feet. Branching is profuse throughout but especially in the 10 inches next to the surface. Roots from the base of the stem and from the taproot extend horizontally but deviously in the second to the eighth inch of soil to distances of 12 to 24 inches. With the numerous branches they constitute the bulk of the absorbing system which is distinctly superficial.
Thus the general root habit of the kidney bean is not greatly unlike that of the pea although the lateral spread and depth of penetration are somewhat greater and the deeper soil, just beneath the plant, somewhat more thoroughly occupied.