164 THE EVOLUTION OF PLANTS 



they grow to a great size and fill the sporangium, 

 which is often actually larger than in the former 

 case (see fig. 18). Sometimes, even the four do 

 not all develop and the number of megaspores 



Fig. 18. Sdagindla. Section of sporophyll, ligule and 

 megaspprangium, showing three megaspores, with the pro- 

 thallus in two of them. After Miss Lyon. X about 40. 



in the sporangium may be reduced to three, 

 two, or even one. The diameter of the mega- 

 spores is commonly about ten times that of the 

 microspores, which is equivalent to a proportion 

 of 1000: 1 in bulk; sometimes the difference is 

 greater. 



