436 THE VASCULAR PLANTS 



thing which has involved the evolution of the pollen tube. 

 If the megaspores or the female gameto- 

 phytes which they produce were cast 

 out into the world instead of being 

 . confined within the tissues of the 



riG. 227. — Seed of 



pine. Note that ovules, there would be no need for 

 the embryo has sev- pollen tubes. This habit of getting the 



eral cotyledons. It . 



lies embedded in the male gamete to the female gamete 

 endosperm. Note by means of a tube is called siphonogamy, 



the thick testa. . i • 1 • j • 



a very expressive term which indicates 

 that the sperm is " siphoned " to the egg. 



F. Why Two Sets of Names ? — We have said that the 

 megasporangium of gymnosperms is the same thing as the 

 ovule. This is also true of angiosperms. All ovules are 

 megasporangia, though of course all megasporangia are 

 not ovules. (Those which occur among pteridophytes 

 are not ovules and Selaginella is not the only member of 

 the pteridophytes which produces megasporangia.) 



Also we have referred to pollen as microspores. We 

 may now carry this doubling of terms a little farther. Let 

 us see how it applies to other parts of flowers. You 

 can see that a stamen is a microsporophyll, and the pollen 

 sacs it bears are microsporangia. The carpels are mega- 

 sporophylls, the megasporangia which they bear being 

 called ovules. 



All this is confusing. Surely it would be better to have 

 only one name for each of these kinds of structures. The 

 trouble is that these organs of flowers all received names 

 long before their relationships to organs of lower plants 

 were understood. Pollen, ovule, stamen, and carpel are 

 terms used long before microspores and megaspores were 



