I2 4 



Plant Life and Evolution 



FIG. 14 



Comparison of the gametophytes of heterosporous Pteri- 

 dophytes and Gymnosperms. 



A Germinating megaspore of Selaginella, showing the en- 

 closed female gametophyte, $, protruding from the ruptured 

 spore apex. 



B Section of a microspore of Isoetes, showing the very 

 much reduced male gametophyte within, v, vegetative cell of 

 gametophyte ; two sperms can be seen in the section. 



C Section of megasporangium (ovule) of a pine. A single 

 megaspore is present, within which is the female gametophyte, 

 g, with the archegonia, ar. Two microspores (pollen-spores) 

 are present in the chamber at the apex of the ovule. These 

 have sent down the pollen tubes. 



D Section of the ripe megasporangium (seed). Within the 

 hard shell is the gametophyte, g, enclosing the young sporo- 

 phyte, em, derived from the egg. 



E Microspore (pollen-spore) of Cycas; from the anther- 

 idial cell, an, two large ciliated sperms are developed. 



a study of their development shows that their 

 essential structure is very much like that of the 



