SPERMATOPHYTES 



141 



The position of the ovule within the ovary cavity has 

 certain features that must be noted. The ordinary entrance 

 to the nucellus is through the micropyle, and therefore the 

 position of the micropyle is important. In some cases, the 

 ovule arises from the bottom of the ovary cavity (or near it) 

 and grows directly away from the wall of the ovary, so that 

 the micropyle is as far from the wall as it can be (Fig. 112, A), 

 which is a relatively unfavorable position. In most Angio- 

 sperms, however, the ovule or its little stalk (funiculus) curves 

 in growing, so that the micropyle is 

 brought relatively near the wall of the 

 ovary (Fig. 112, B and C). That this 

 position is a favorable one is evident 

 when it is understood that the pollen 

 tube grows along the wall of the ovary 

 and enters the ovule by way of the 

 micropyle. In examining ovules it 

 will be found that most of them are 

 not straight, but are curved in various 

 ways, and the curving means a more 

 favorable relation of the micropyle to 

 the entrance of the pollen tube. 



85. The gametophytes. It was 

 stated ( 73, p. 123) that among the Gymnosperms the male 

 gametophyte is represented by a few cells developed by the 

 microspore (pollen grain) and remaining within it, and the 

 female gametophyte by a larger group of cells developed by 

 the megaspore (within the ovule) and remaining within it. 

 The same statements are true of the Angiosperms, and the 

 gametophytes are still more reduced in the number of cells. 



The male gametophyte (within the pollen grain) consists 

 usually of three cells, often represented only by three nuclei 

 (Fig. 113). One of them is the nucleus associated with the 

 development of the pollen tube, and hence is called the tube 

 nucleus; the other two are the sperms (represented either 



FIG. 113. Pollen grain 

 (microspore) containing 

 the male gametophyte 

 which consists of three 

 cells or nuclei ; the up- 

 permost nucleus is the 

 tube nucleus ; the two 

 cells, each containing a 

 nucleus, are the sperms. 



