lo THE PHYSIOLOGY OF PLANTS 



elongation, already alluded to, of the cells lying behind the 

 root-apex, which have lost their meristeniatic character and 

 have become transformed into the permanent tissues of the 

 root. By their growth in length they push the apex forward, 

 as the older portion of the root is firmly fixed in the soil by 

 the root-hairs, and cannot therefore be forced back. But how 

 could a soft root-tip, composed as it is of the most delicate 

 cells, be forced between the sometimes very sharp particles of 

 the soil without sustaining severe injury ? Nature has guarded 

 against any such damage by protecting the root with a cap, 

 the oldest cells of which are turned towards the outside, and 

 protect the growing cells at the interior from any harm. 



In the case of other organs (leaves, for instance) we can see 

 how their peculiar development is an adaptation to the sur- 

 rounding conditions, and we might presuppose the same for 

 roots ; nor shall we go far wrong if we assume that the pro- 

 duction of the root-cap is a necessary adaptation of the root 

 to its conditions of growth, and that it has thus gradually been 

 evolved. 



The centre of growth, the elonfratin" and the absorbing regions 

 of the root, are all confined to the root-tip, and consequently the 

 conclusion we arrive at from our preceding observations is that 

 the first eondition of a rational method of cultivation is to 

 preserve and increase the number of root-tips. 



(c.) The Conducting Portion of the Root. 



After having examined the seat of growth in the root, and 

 having determined the region of absorption, it is necessary to 

 look for the tissue through which the absorbed water with its 

 dissolved food substances is conducted. This tissue is the 

 woody tissue of older portions of the root, which, as can easily 

 be seen in any root of a tree, forms a hard central cylinder, 

 and is surrounded by a soft cortical layer. To form the central 

 cylinder, the elements which we have mentioned as G in Fig. i 

 become more and more numerous, and group themselves into 

 thick strands arranged in a ring. These strands are termed the 

 "vascular bundles." In both Mono- and Dicotyledons they 

 consist of various elements, some of which are of the nature of 



