PTERIDOPHYTES 93 



are constructed so that the loss of water may not be greater 

 than the supply will be considered later. 



56. The stem. An independent sporophyte has not 

 only a vascular system and leaves, but also a stem ; in fact, 

 the presence of leaves implies a stem to bear them. The most 

 important fact about a stem is that it bears leaves and exposes 

 them to the air and the sunlight. In proportion as stems be- 

 come taller, the better are the leaves exposed ; and in propor- 



FIG. 73. Cross-section of the central cylinder of the stem (rootstock) of a fern ; the 

 cylinder is solid, the large water-conducting vessels (xylem) being at the center. 



tion as the stems become branched, more leaves can be pro- 

 duced and exposed. 



As the stems carry the leaves up into the air and sunlight, 

 they must also supply them with water, and this means that 

 the vascular system of the stem must connect with the vas- 

 cular system (vein system) of the leaves. In the stem the 

 vascular system is organized as a central cylindei^so that it 

 is called the vascular cylinder (Figs. 73 anoT74)7 This cyl- 

 inder not only conducts water, but also gives rigidity to the 



