GYMNOSPERMS 



435 



familiar. The cones usually hang to the tree for a year 

 or more after they are pollinated, generally shedding their 

 winged seeds before they 

 drop. Meanwhile the em- 

 bryo grows inside the seeds. /> . \ ; 

 The embryos of all our com- / | // /^n y 

 mon gymnosperms are dif- 

 ferent from those of angio- 

 sperms in that they have 

 several cotyledons instead of 

 only one or two. (See Fig- 

 ure 22J.) 



E. The Seed. — You re- 

 member that in discussing 

 Selaginella (page 430) we 

 spoke of the retention of 

 the megaspore, and said that 

 this was responsible for the 

 evolution of the seed. You 

 can now see how this is. 

 Evidently if the young pine 

 cones shed their megaspores, 

 there would be no seeds. 

 The megaspores would be 

 cast out from the ovules just 

 as the pollen grains are cast 

 out from the pollen sacs, 

 and seeds would be no more 



Fig. 226. — A, diagram of a lengthwise 

 section through a megasporophyll of 

 pine ; the shaded portion is the young 

 female gametophyte lying within the 

 ovule {megasporangium). B, length- 

 wise section through the ovule only; 

 this ovule is older than the one repre- 

 sented in the other figure ; two pollen 

 tubes are seen entering it; they are 

 approaching the two eggs which lie at 

 the lower end of the female gameto- 

 phyte, which is represented by the 

 long oval. 



produced than they are in 



the pteridophytes. Also we can see that this retention of 



the megaspores within the ovules which produce them is a 



