770 REMARKABLE PROPERTIES OF ROOTS. 



It has been shown that where stems are drawn into the ground, it is by 

 means of the roots. After the growth in length of a root is completed, it shortens, 

 in some instances only 2-3 per cent, in other cases as much as 20-30 per cent, 

 i.e. almost a third of its entire length. The shortening depends upon alterations 

 in the turgidity of the cells connected with an absorption of water. While the 

 cells of that portion of the root which is still growing elongate by increased 

 turgescence, those of the fully-formed root become shorter and broader in con- 

 sequence of the increase in their turgidity. The parenchymatous cells in fully- 

 formed roots become broader at the expense of their length in consequence of 

 the increased turgescence produced by the absorption of water, and the natural 

 result is a shortening of the whole tissue-body. This contraction of the mature 

 root-portion exerts a tension on both ends. At the lower end of the fully-formed 

 part of the root is the still immature portion growing downwards, whilst at the 

 upper end that part of the stem from which the root originated. Above the 

 downwardly-directed point the immature part of the root is equipped with hair- 

 like absorbent cells, and these are closely united to the surrounding soil. In this 

 way a resistance is afforded which the strain of the contracting part of the root 

 cannot overcome. And since, as already stated, the cells at the growing end of 

 the root are lengthened by turgescence, the tissue is extended, and the root-tip, 

 in spite of the strain operating from above, continues to penetrate into the 

 ground. The strain, therefore, has no effect in this direction. But it is otherwise 

 with the pull exercised on the stem by this contracting of the root. There is no 

 powerful resistance here to be overcome, and consequently the part of the stem 

 in question, whether the hypocotyl of the embiyo, the end of the epicotyl, or a 

 node from the middle or end of the leafy foliage-stem, is drawn down into the soil. 



This remarkable planting of course only occurs where the roots grow down 

 vertically into the ground, and as remarked, it is most noticeably observed in 

 species whose subterranean stem and root-structures store up reserve materials. 

 Roots which run horizontally below the surface of the ground are not adapted 

 to influence the stem in the manner indicated. On the contrary, in certain 

 circumstances, these are able to effect an elevation of the stem. This happens 

 especially in trees with thick woody roots, e.g. in pines and firs, oaks and chestnuts, 

 and is to be explained in the following simple way. The first embryonic root 

 growing down vertically into the ground soon dies off, or its development, 

 especially its increase in length, is greatly retarded, and much more vigorous 

 roots develop from it, or from the lowest part of the erect hypocotyl. These 

 spread out in a horizontal direction under the surface of the ground. They 

 usually radiate out in all directions forming a whorl at the base of the erect 

 stem, as can be plainly seen in pines uprooted by a devastating storm. These 

 horizontal roots at first have only a slight thickness, but their diameter in- 

 creases with age, and the successive layers of wood in them form annual rings, 

 just as in stems. Now these subterranean roots, in addition to resisting the 

 pressure of the surrounding soil, actually exercise a considerable lateral pressure 



