DEVELOPMENT OF PLANTS 



389 



ter nuclei move to each end of the spore (Fig. 270, A). Each 

 of these nuclei divides twice, forming four nuclei at either end 

 of the spore (Fig. 270, B, C). This generally completes the 

 nuclear divisions in the gametophyte. Only a few cases are 



FIG. 269. Development of the megasporangium and megaspore: A, sec- 

 tional view of the pepper-grass, Lepidium. This flower has two cohering 

 sporophylls, only a portion of the right-hand one being shown, s, stigma 

 with protruding cells to receive the microspores; mg, megasporangium con- 

 taining a single spore mother cell, me. Two integuments, i, are growing up 

 about the sporangium. B, later stage of development, the megasporangium, 

 mg, becoming inverted and completely covered by the integument. The 

 mother cell of A has formed four daughter cells in series and not in tetrads 

 as in the Pteridophyta. The innermost cell of the series, ms, only matures 

 as a megaspore; m, micropyle;/, stalk or funiculus of sporangium. 



known where a larger number of nuclei are formed compare 

 with the gymnosperms. A nucleus from each of these groups, 

 called the polar nuclei, now approach each other and fuse, 

 forming a single large nucleus that is usually called the endo- 

 sperm nucleus. This growth of seven cells represents the female 

 gametophyte. The three outer or micropylar cells are not pro- 



