428 



THE VASCULAR PLANTS 



The sporangia which produce megaspores are called 

 megasporangia, and the sporophylls which bear them are 



called megasporo- 

 phylls. Similarly we 

 speak of the micro- 

 sporangia and micro- 

 sporophylls. All 

 these facts are also 

 true of seed plants 

 and the same terms 

 are used to describe 

 them. 



This matter is 

 made easier to un- 

 derstand by studying 

 the pictures. In 

 Figure 220 you see 

 the two kinds of 

 sporophylls, the two 

 kinds of sporangia 

 which they bear, 

 and the two kinds of 

 spores which are pro- 

 duced. The mega- 

 sporangium contains only four megaspores. 



The life history of Selaginella may be indicated by the 

 following formula in which S stands for the sporophyte 

 generation and G for the gametophyte generation. 



o t- h'l /■"^g^^P'"'°P'^yl's megasporangia megaspores 



N^microsporophylls microsporangia microspores 



female G egg \ 



Zoospore c 



male (j sperm/ •^ 



Fig. 220. — The two kinds of sporophylls of Sela- 

 ginella. The one at the left bears a sporangium 

 containing many small spores (microspores) ; the 

 one at the right bears a sporangium containing 

 just four large spores (megaspores). The mega- 

 spores and microspores are shown below the 

 sporophylls which bear them; note the differ- 

 ence in size. 



