SPERMATOPHYTES 123 



more or less extended tube. The covering is the integument, 

 and the tube is the micropyle (" little gate "). Within the 

 covering is the body of the ovule (nucellus), with its tip at 

 the base of the open micropyle. If the ovule is a mega- 

 sporangium, it must contain megaspores, and these are found 

 in the nucellus. Several megaspores start, but only one 

 grows, and it becomes so large that it looks like a cavity in 

 the middle of the nucellus (Fig. 98, A). The peculiarity of 

 this megasporangium (the ovule) is not that it has only one 

 megaspore, or that the megaspore is so large, but that it is 

 never shed, that is, it never escapes from its megasporangium. 

 The fact that this megaspore is retained in its sporangium is- 

 the reason why the ovule becomes a seed. 



72. The stamen. There is nothing peculiar about the 

 stamen (microsporophyll), except that among Gymnosperms 

 it becomes more and more unlike a leaf in appearance. In 

 some Cycads it appears as a flat blade bearing sporangia 

 (pollen sacs) on its under surface ; in pines the blade becomes 

 short-stalked (Fig. 97, B) ; and in many other Gymnosperms 

 the stalk becomes elongated and the blade reduced to a plate 

 or knob bearing the pollen sacs. When two regions of a 

 stamen are distinguishable as a stalk region and a pollen-sac 

 region, the former is called the filament, and the latter the 

 anther. These names are often convenient in describing 

 stamens, but they only mean that some microsporophylls 

 have stalks distinct from the sporangium-bearing region. 



73. The gametophytes. In the preceding chapter ( 63, 

 p. 110) it was stated that the gametophytes of heterosporous 

 Pteridophytes do not emerge from the spores that produce 

 them. Of course all Seed-plants are heterosporous, and, 

 therefore, just as in heterosporous Pteridophytes, the male 

 gametophyte develops within the microspore (pollen grain), 

 and the female gametophyte develops within the megaspore 

 which is retained within the megasporangium (ovule). 

 This means that in Seed-plants the gametophytes are in- 



