124 



MORPHOLOGY 



leaves), each one having at base a mass of sporogenous tissue (sporangium}. 

 This final structure is the simple Lycopodium body just described. It is the 

 so-called Selago type, in which all or nearly all the leaves are sporophylls, and 

 hence practically the whole body is a strobilus. It must be understood that this 

 proposed origin of the Lycopodium sporophyte is simply a theory, but it is a 

 very suggestive one. 



The more complex sporophytes of Lycopodium are branching bodies 

 (fig. 265). There is a gradual sterilization of the lower sporophylls, 



which thus become simply foliage 

 leaves. Finally the sporophyte 

 becomes differentiated into two 

 distinct regions : that bearing 

 foliage leaves and that bearing 

 sporophylls. The sporophylls 

 finally become quite different 

 in appearance from the foliage 

 leaves and are organized into 

 a compact strobilus, which is 

 sometimes separated from the 

 branching leafy body by a long 

 stalk bearing only rudimentary 

 leaves (fig. 266). 



Vascular system. The anat- 

 omy of the stem emphasizes 

 further the primitive character 

 of this sporophyte. A cross 

 section shows two regions : the 

 cortex, an outer region of living 

 cells ; and the central cylinder 

 or stele, in which the vascular 

 system (conducting system) is 

 developed. The vascular system 

 has been f o" nd to be of great 

 importance in any study of the 

 evolution of vascular plants, 



and, therefore, the outline of its history must be indicated. In the 

 simpler lycopodiums, or in young stems, the vascular system of the 

 stem forms a solid axial cylinder, in which the xylem (the group of 

 water-conducting vessels) is completely surrounded by the phloem 



sporophyte showing 



tween foUage region and strobilus. 



u 



be- 



