520 General Botany 



The members of the outer cycle, or calyx, are usually green, but 

 in the lily family they are often similar to the corolla in form and 

 color. 



The microspores are produced in four sporangia which make 

 up the anther. At maturity the sporangia open in pairs so that 

 they appear as two pollen sacs. The young pollen grain, or 

 microspore, consists of a single cell ; when shed, the microspore 

 has divided internally and the mature pollen contains two or 

 three cells. 



The megaspores arise within megasporangia, which are usually 

 called ovules. These are variously arranged inside the carpels 

 (megasporophylls). There may be one or many hundred ovules 

 within each ovulary. Each ovule consists of a nucellus inclosed 

 by two integuments, except for one small opening, the micropyle. 

 Within the nucellus four megaspores form, but only one of them 

 matures. 



The female gametophyte. The megaspore germinates, or con- 

 tinues to enlarge within the nucellus, and ultimately forms 

 the embryo sac or female gametophyte. This consists of seven 

 cells, one of which is the egg nucleus, and near the center of the 

 embryo sac is the fusion nucleus. The female gametophyte, 

 then, has been greatly simplified in comparison with that of 

 gymnosperms. 



The male gametophyte. Previous to the time of pollination 

 the microspore has divided internally, forming two nuclei — one, 

 the tube nucleus ; the other, the generative nucleus. The latter 

 may have divided a second time, forming the two sperms. The 

 male gametophyte is, therefore, reduced to its simplest form. 



Fertilization. After the pollen has reached the stigma and 

 germinated, the pollen tube penetrates the tissues of the style 

 and enters the nucellus of the ovule through the micropyle. As 

 the pollen tube elongates, the tube nucleus maintains a position 

 near the tip and the two sperms follow just behind it. 



When the pollen tube reaches the embryo sac, it discharges 



