44 



STRUCTURAL BOTANY 



II. INTERNAL STRUCTURE OF THE SPOROPHYTE 



A. THE VEGETATIVE OKGANS 



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 polystelic, i.e. all the 

 later - formed part of the 

 stem contains a number of 

 distinct steles, or cylinders ; 

 in fact the simple monostelic 

 structure is only found at 

 the very base or oldest 

 portion of the stem; this 

 part soon dies away alto- 

 gether, so the whole stem of 

 apex; i, i, bases of leaves ; si5, a Male Fern, as soon as it is 



stem ; g. leaf gap. Natural size. . . , , , , . , , . , , 



(After Sachs.) 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 through- 

 out life. We will now confine our attention to the 

 mature stem of the Male Fern. 



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

 a lens, the transverse section of the stem, we at once 



