THE SHOOT 43 



many rows of lateral roots as there are wood strands. 

 A little group of the cells become marked out by be- 

 coming meristematic, and dividing chiefly by tangential 

 walls, so that soon a little mass seems to be growing 

 outwards. It can shortly afterwards be seen to have a 

 central plerome covered by a periblem and dermatogen, 

 which behave just like those of the parent root. The 

 cells of the cortex which lie in front of the new root 

 branch are gradually digested and eaten by the latter as 

 it grows outwards and finally penetrates to the exterior 

 (Fig. 17). 



The cells of the root cap are continually being worn 

 away by contact with the soil. The cap is added to all 

 the while by the dermatogen behind it. 



CHAPTER V 



THE CHARACTERISTIC FEATURES OF THE SHOOT 



THE work which falls upon the shoot portion of the 

 plant is very different from that discharged by the roots, 

 being very largely the construction of the organic sub- 

 stance which serves as food, not only for the plant itself 

 but for the world in general. To understand this con- 

 struction we must consider the absorption of carbon 

 dioxide, the utilisation of certain amounts of the water 

 and mineral constituents furnished by the roots, and 

 the evaporation of the surplus water. The work in- 

 volves certain minor or subordinate duties connected 

 with the distribution of the food after its formation. 



The important questions of the breathing of the plant 

 and the maintenance of a suitable temperature in its 

 different parts must also engage our attention. 



The form and composition of the shoot need careful 

 study from these points of view, but these are not all. 

 The relation of its structure, internal as well as external, 



