PTERJDOPHYTA 



243 



prismatic layer contains any secondary phloem, as has been claimed, is a 

 matter of considerable uncertainty. 



As cambial activity continues, the outer tissues of the stem are con- 

 stantly sloughed ofT, the cortex finally being made up wholly of secondary 

 tissue. Numerous leaf traces arise from the stele of the stem, one going 

 to each leaf. In the lower part of the stem (rhizophore) root traces sim- 



■ . r. J --' 





>*#V 



■■.'(?:*-■•■■; 





« # . . . 





Fig. 200. Cross section of the central portion of the stem of Isoetcs howdlii, X 100. The 

 primary xylem, in the center, is surrounded by a narrow zone of primary phloem enclosed 

 by tissues derived from the cambium. 



ilarly pass from the stele to the roots. The root traces contain much more 

 xylem and phloem than the leaf traces. 



Sporangia. Isoetes, like Selaginella, is heterosporous, but nearly all the 

 leaves are sporophylls. As a rule, the outer leaves bear megasporangia 

 and the inner ones microsporangia, while the few central leaves are sterile. 

 The sporangia, mostly -4 to 7 mm., but up to 10 mm. in length, are larger 

 than those of any other living pteridophyte. They are solitary and adax- 

 ial, each one being sunken in a cavity at the base of the sporophyll just 

 below the Hgule (Fig. 201 A, B). Each sporangium is partially or com- 

 pletely overgrown by a membrane called the velum. The microsporangia 

 may produce as many as 300,000 microspores, the megasporangia up to 

 300 megaspores. In each of the two kinds of sporangia sterile plates, 



