ANGIOSPEEMS 



349 



the evolution of the seed and the seed habit. The more important 

 of these changes are represented graphically in Figs. 205 and 206. 

 In these figures it may be seen that the microspores have remained 

 quite similar in form, size, and structure, since their function has 

 not changed. In the angiosperm, as in Selaginella, the microspores 

 still serve the function of bearing the male gametes to the female 

 gametes, and are hence small, light, and highly protected cells, pro- 

 duced in great numbers by the microsporangia to guard against 

 waste in distribution 



oelagtnelut spruce 



by wind or insects. 



The male gameto- 

 phyte has been grad- 

 ually reduced, as was 

 explained in a previous 

 paragraph, until it is 

 represented in the an- 

 giosperm by a single 

 generative cell. The 

 megaspores (Fig. 206) 

 have become reduced 

 to a single megaspore 

 in each megasporan- 

 gium, both in the gym- 

 nosperms, represented 

 by the spruce, and in 

 the angiosperms, rep- FIG. 206. Diagram showing the homologous, or 

 resented by the man- corresponding, parts of megaspores, megasporan- 



g^metophytes, and sporophytes of Selaginella, 

 spruce, and mandrake 



a-c, megaspores; d-f, gametophytes ; g-i, gameto- 

 ph ytes and sporophytes. A and i are seeds 



drake. The important 



advance made by the 

 -, , P-, 



latter groups of plants 



over Selaginella and 

 similar heterosporous Pteridophyta is that of retaining the mega- 

 spore permanently in the sporangium. The megatporanpium thus 

 becomes indehiscent and is shed from the mother plant and dis- 

 seminated with the contained megaspore and embryo. The female 

 gametophytes remain cellular structures produced by the germina- 

 tion of the megaspore in both Selaginella and the spruce. In Selagi- 

 nella, however, the gametophyte and archegonia are still exposed 

 by the opening of the megasporangium and spore. In the indehis- 

 cent megasporangium of the spruce the cellular gametophyte is 

 retained permanentlv in the spore within the megasporangium, but 



