=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.