438 THE VASCULAR PLANTS 



the testa is, of course, the modified outer part of the ovule ; 

 therefore it belongs to the old sporophyte generation. (It 

 is when the testa is formed that the ovule ceases to be an 

 ovule ; when the testa appears, we speak of this structure 

 as a seed.) The endosperm is the female gametophyte, 

 while the embryo is the new sporophyte. Thus we find 

 in seeds three generations represented. It is evident that 

 we do not understand seeds if we do not understand these 

 generations, and it is evident that we cannot understand 

 these generations without understanding the lower plants. 

 We see now that seeds are a sort of climax in the evolu- 

 tion of plant reproduction. We see that the gradual dif- 

 ferentiation of reproductive structures has led finally to 

 seeds, which are the most familiar and the most successful 

 of all plant reproductive structures. We see more than 

 ever that we cannot understand seed plants without un- 

 derstanding something of the lower plants from which they 

 have evolved. We need to have in mind a clear picture of 

 this evolution. 



/. Summary of the Life History of a Gymnosperm. - 

 The sporophytes of gymnosperms are composed of roots, 

 stems, and leaves ; commonly they are that form of plant 

 which we call a tree. These sporophytes bear certain 

 short branches called cones. These cones are aggregations 

 of sporophylls; that is, they are strobili. The cones are 

 of two kinds. One kind bears microsporophylls, the 

 other, megasporophylls. On the under surface of the 

 microsporophylls we find structures called pollen sacs ; 

 these are microsporangia. On the upper surface of the 

 megasporophylls we find structures called ovules ; these 

 are megasporangia. The pollen sacs contain pollen grains 



