122 



THE BIOLOGY OF A PLANT. 



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 



iv. 



FIG. 54. (After De Bary.) Sieve-tubes from the rhizome of Pteris aquilina, show- 

 ing : A, the end of a member of a sieve-tube ; B, part of a thin longitudinal sec- 

 tion. The section has approximately halved two sieve-tubes, S 1 and S 2 , 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 1 , on the 

 contrary, abuts by a smooth non-plated surface upon parenchymatous cells 

 which are seen through it. ?<\ sections of walls bearing sieve-pits ; .r, section of 

 a non-plated wall abutting upon parenchyma. 



stein and branches. They arise endogenously from the main 



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



j ~ 



and not (as in the case of the false roots or rhizoids of the pro- 

 thallium, shortly to be described) by elongation of superficial 

 cells of the epidermis. True roots, of which those of Pteris are 

 good examples, arise always as well from the fundamental and 

 fibro- vascular regions, and include all the systems found in the 

 stem itself. Hence cross-sections of Pteris roots differ but 

 slightly from those of the stem or the branches, and the root in 

 general is clearly a member of the plant body. As in all true 

 roots, the free end is covered by a special boring tip called the 



