THE STAMEN AND THE FORMATION OF POLLEN 379 



with accessory parts. The gymnosperms do not have the exact 

 equivalents of fruits, although the berry-like structures of the 

 yew appear at first glance to be similar and the cone is, of 

 course, a protective structure for the seeds. True fruits, as the 

 term is used when applied to the angiosperms, are seed cases of 

 various forms, structures which are sometimes merely protec- 

 tive, and sometimes fleshy and attractive to animals for food. 

 They are described in Chapter xvi. 



The pistil distinguishes the angiosperms from the gymno- 

 sperms, and is a more important feature of the angiosperm 

 flower than the perianth, which is frequently inconspicuous, 

 and sometimes wholly or almost wholly absent. But the pistil 

 in combination with a showy perianth of some peculiar and 

 specialized form gives the highest types of flower structure. 

 The most important of these are discussed in Chapter xm. 

 This account will only describe the stamens and carpels in their 

 functions as spore-producing organs developing microspores 

 (pollen) and megaspores (embryo sacs). 



359. The stamen and the formation of pollen. The parts 

 of a stamen are described in Sees. 171 to 173. Pollen formation 

 takes place generally in four regions of the anther, which become 

 pollen sacs, or loculcs (Fig. 302, A). The cells of these regions 

 develop the pollen grains in groups of four, or tetrads (Fig. 302,7?), 

 and are consequently pollen mother cells. This process is iden- 

 tical with that of spore formation in the pteridophytes and 

 bryophytes. 1 The pollen mother cell is a spore mother cell, and 

 the pollen grain a spore, or more exactly a microspore. 



The pollen sacs are . sporangia, and like the sporangia of 

 the horsetails, lycopods, Selaginella, and the pollen sacs of the 



1 As in the case of the gymnosperms, the count of the chromosomes during 

 pollen formation shows it to be a period of chromosome reduction, when 

 the sporophyte generation passes over to the gametophyte, as explained in 

 Sees. 334 and 335. Thus 24 chromosomes have been counted in various tissues 

 of the lily plant, but only 12 appear in the nuclear divisions in the pollen 

 mother cell (Fig. 302, B). These cells, it may be remarked, are exceedingly 

 good subjects for the study -of nuclear division 



