94 



ELEMENTARY STUDIES IN BOTANY 



stem, so that in most cases it stands upright. The move-^ 

 ment of water up the stem, through the vascular cylinder, is 

 commonly spoken of as the " ascent of sap," the " sap 

 being water on its way to the leaves. 



It must not be supposed that all stems are upright, for in 

 many Olub-mosses they are prostrate, but as they elongate 

 they produce and display many leaves. Nor must it be sup- 

 posed that all stems are 

 above ground, for in the 

 most common Ferns the 

 stem is underground, but 

 it sends its leaves above 

 ground. These under- 

 ground stems are usually 

 mistaken for roots, but 

 they can always be recog- 

 nized as stems by the/ 

 fact that they produce/ 

 leaves and by the kind 

 of vascular cylinder they 

 possess. 



57. The root. --An 

 independent sporophyte 

 has not only a vascular 

 system, leaves, and a stem, but also roots. The leaves 

 need water, which the stem supplies, but roots must receive 

 water from the soil and supply it to the stem. Thus, 

 the vascular system is a water-conducting system connecting 

 the roots with the leaves, through the stem. No one of the 

 four structures mentioned as belonging to an independent 

 sporophyte is independent of the others. The presence of 

 leaves implies a vascular system, a stem, arid a root ; and 

 so for each structure in turn. They all belong together as 

 parts of one machine. How the roots receive water from 

 the soil will be considered later. 



FIG. 74. Cross-section of the central cylinder 

 of the stem of a fern ; the water-conduct- 

 ing vessels form a hollow cylinder inclosing 

 pith. 



