244 



STRUCTURE AND VITAL PROCESSES OF PLANTS 



through the scales. 



Fig. 224. 

 Tubers of Potato. 



Its axis suddenly turns up and gives rise 

 to an overground shoot, whilst the creep- 

 ing stem is continued in its original 

 direction by a new branch-shoot, side-shoots 

 also being given off occasionally. 



The Bulb (see Onion, p. 154) is a flat, 

 solid stem with large succulent overlap- 

 ping scales and leaves. 



The Tuber (see Potato, p. 91) is a 

 fleshy, roundish mass with small scales 

 (or scars of such) and small buds in their 

 axils. Some tubers, e. g., of Taro, have 

 one bud only at their apex. 



B. Steins as Channels for the Ascending and 

 the Descending Sap. 



We have heard of the water-current from the root upwards 

 to the leaves where the water and the mineral substances 

 dissolved in it are used for the formation of starch and other 

 material required for the building up of tissues. We have also 

 learned that the superfluous water is evaporated through the 

 leaves. Hence there must be a continuous upward flow of water. 



On the other hand, we have seen that the substances elabo- 

 rated in tlie leaves cannot remain useless in the cells of the 

 leaves, but must be carried to those places where new tissue is 

 formed, such as the tips of the roots. Hence, there must be a 

 continuous downward flow of licjuid food stutt'. 



We shall, therefore, now proceed to make a study of the 

 stem as a channel for the ascending and descending sap. 



1. General Structure and Leading Tissues of Stems. 



A transverse section of a herbaceous stem, examined by 

 means of a hand-lens, exhibits the following leading tissues: In 

 the outer layer of cells (flg. 225, E) we recognize the epidermis 



