Fig. 110.
Root of germinating pumpkin, showing region of elongation just back of the tip.
249. The region of elongation.—While the root has elongated, the region of elongation is not at the tip of the root. It lies a little distance back from the tip, beginning at about 2mm from the tip and extending over an area represented by from 4-5 of the millimeter marks. The root shown in [fig. 110] was marked at 10 a.m. on July 5. At 6 p.m. of the same day, 8 hours later, growth had taken place as shown in the middle figure. At 9 a.m. on the following day, 15 hours later, the growth is represented in the lower one. Similar experiments upon a number of seedlings give the same result: the region of elongation in the growth of the root is situated a little distance back from the tip. Farther back very little or no elongation takes place, but growth in diameter continues for some time, as we should discover if we examined the roots of growing pumpkins, or other plants, at different periods.
250. Movement of region of greatest elongation.—In the region of elongation the areas marked off do not all elongate equally at the same time. The middle spaces elongate most rapidly and the spaces marked off by the 6, 7, and 8 mm marks elongate slowly, those farthest from the tip more slowly than the others, since elongation has nearly ceased here. The spaces marked off between the 2-4 mm marks also elongate slowly, but soon begin to elongate more rapidly, since that region is becoming the region of greatest elongation. Thus the region of greatest elongation moves forward as the root grows, and remains approximately at the same distance behind the tip.
251. Formative region.—If we make a longitudinal section of the tip of a growing root of the pumpkin or other seedling, and examine it with the microscope, we see that there is a great difference in the character of the cells of the tip and those in the region of elongation of the root. First there is in the section a V-shaped cap of loose cells which are constantly being sloughed off. Just back of this tip the cells are quite regularly isodiametric, that is, of equal diameter in all directions. They are also very rich in protoplasm, and have thin walls. This is the region of the root where new cells are formed by division. It is the formative region. The cells on the outside of this area are the older, and pass over into the older parts of the root and root cap. If we examine successively the cells back from this formative region we find that they become more and more elongated in the direction of the axis of the root. The elongation of the cells in this older portion of the root explains then why it is that this region of the root elongates more rapidly than the tip.
252. Growth of the stem.—We may use a bean seedling growing in the soil. At the junction of the leaves with the stem there are enlargements. These are the nodes, and the spaces on the stem between successive nodes are the internodes. We should mark off several of these internodes, especially the younger ones, into sections about 5 mm long. Now observe these at several times for two or three days, or more. The region of elongation is greater than in the case of the roots, and extends back farther from the end of the stem. In some young garden bean plants the region of elongation extended over an area of 40 mm in one internode. See also Chapters [38], [39].
253. Force exerted by growth.—One of the marvelous things connected with the growth of plants is the force which is exerted by various members of the plant under certain conditions. Observations on seedlings as they are pushing their way through the soil to the air often show us that considerable force is required to lift the hard soil and turn it to one side. A very striking illustration may be had in the case of mushrooms which sometimes make their way through the hard and packed soil of walks or roads. That succulent and tender plants should be capable of lifting such comparatively heavy weights seems incredible until we have witnessed it. Very striking illustrations of the force of roots are seen in the case of trees which grow in rocky situations, where rocks of considerable weight are lifted, or small rifts in large rocks are widened by the lateral pressure exerted by the growth of a root, which entered when it was small and wedged its way in.
254. Zone of maximum growth.—Great variation exists in the rapidity of growth even when not influenced by outside conditions. In our study of the elongation of the root we found that the cells just back of the formative region elongated slowly at first. The rapidity of the elongation of these cells increases until it reaches the maximum. Then the rapidity of elongation lessens as the cells come to lie farther from the tip. The period of maximum elongation here is the zone of maximum growth of these cells.
Fig. 111.
Lever auxanometer (Oels) for measuring
elongation of the stem during growth.
255. Just as the cells exhibit a zone of maximum growth, so the members of the plant exhibit a similar zone of maximum growth. In the case of leaves, when they are young the rapidity of growth is comparatively slow, then it increases, and finally diminishes in rapidity again. So it is with the stem. When the plant is young the growth is not so rapid; as it approaches middle age the rapidity of growth increases; then it declines in rapidity at the close of the season.