122 PH YSIOL OG Y. 



croscope, 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 pro- 

 toplasm, 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 $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 ^omm 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 succu- 

 lent and tender plants should be capable of lifting such comparatively heavy 

 weights seems incredible until we have witnessed it. Very striking illustra- 

 tions 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 



