44 



STRUCTURAL BOTANY 



II. INTERNAL STRUCTURE OF THE SPOROPHYTE 



A. THE VEGETATIVE ORGANS 



1. The Stem 

 a. The Vascular System 



In all the plants with which we have had to do so far 

 (except some of the Selaginellas), the vascular system of the 

 stem evidently constitutes a single cylinder, or stele. In the 

 Male Fern and most other Ferns this is only the case 

 while the stem is still quite young. As the growing- 

 point strengthens, a more complex structure is gradually 



built up, and the stem soon 

 becomes potystelic, i.e., in all 

 the later-formed part of the 

 stem the original stele has 

 broken up into a network of 

 vascular strands or steles ; 

 in fact the simple monostelic 

 structure is only found at 

 the very base or oldest 

 portion of the stem ; this 



FIG. 19.-- Longitudinal section of fc di ^ 



small stem of Male I 1 era. a, J 



apex ; I, I, bases of leaves ; st, gether, SO the whole stem of 



<SJ &** NatUral Sl " a Male Fern, as soon as it is 



once fairly established in the 



ground, is polystelic. This is the case with the majority 

 of Ferns, but a few, such as the Filmy Ferns, retain 

 monostelic structure throughout life. 



If we examine with the naked eye, or with the aid of 

 a lens, the transverse section of the mature stem of the 

 Male Fern, we at once see the cut ends of the steles, 



