80 



THE BIOLOGY OF A PLANT. 



The tracheids and vessels are of great physiological import- 

 ance, being probably the main channels for the flow of sap. 

 Sap is water holding various substances in solution. The water 

 enters by the roots, flows principally through the walls of the 

 vessels and tracheids, and not through their cavities, which are 

 filled with air, and is thus conducted through the rhizome and 

 upwards into the leaves. 



8. Wood-parenchyma cells like those of the phloem-paren- 

 chyma (5) scattered between the vessels and tracheids. 



Branches of the Rhizome. These repeat in all respects the 

 structure of the main stem. They are equivalent members of 

 the underground part, and differ in no wise, excepting in their 

 origin, from the main stem itself. 



Roots. The roots may easily be recognized by their small 

 size and tapering form, and their lack of the lateral ridges of the 



FIG. 41. (After De Bary.) Sieve-tubes from the rhizome of Pteris aquilina, showing : A, the 

 end of a member of a sieve-tube; B, part of a thin longitudinal section. The sec- 

 tion has approximately halved two sieve-tubes, Sj and S, which are so drawn that the 

 uninjured side lies behind. The broad posterior surface of S 2 is seen covered with 

 sieve-plates connecting with another sieve-tube. S^ on the contrary, abuts by a smooth 

 non-plated surface upon parenchymatous cells which are seen through it. w, sections 

 of walls bearing sieve-pits; a-, section of a non-plated wall abutting upon parenchyma. 



stem and branches. They arise endogenously from the main 

 stem or its branches, i. e., by an outgrowth of the internal tissues 



