374 TEXTBOOK OF BOTANY 



In a very young ovulary the ovules are seen as small mounds of cells 

 that have developed from a particular part of the ovulary termed the 

 placenta. In microscopic sections of these young ovules all the cells 

 look alike; but in sections of a slightly older ovule one cell near the tip is 

 seen to be larger than any of the others. This cell is a megaspore mother 

 cell, or megasporocijte. From this cell through successive cell divisions 

 there results a group of four (tetrad) megaspores. Three of the mega- 

 spores ordinarily disintegrate, and only one develops further. 



By this time the ovule has enlarged and its three parts — the stalk, 

 nucellus, and integuments, or coats — can be recognized (Fig. 166). 

 Since the megaspores develop within it, the ovule may be called a 

 megasporangitim. The integuments do not quite enclose the ovule at 

 one end, thus leaving a small pore known as the micropyle. After the 

 first nuclear division the megaspore technicallv becomes a 2-nucleate 

 embryo sac. Through subsequent nuclear divisions the embryo sac finally 

 contains eight^ nuclei, one of which becomes the female gamete, or egg. 

 Two other nuclei unite and form the fusion nucleus. The embryo sac 

 thus becomes 7-nucleate. Of these the egg and fusion nucleus are the 

 important ones. The other five nuclei are usually transient and disinte- 

 grate, though in certain species of plants some of them divide and a 

 small group of cells develops from them; occasionally an embryo may 

 develop from one or more of them. 



The pollen in the stamen now becomes important in our account 

 of sexual reproduction. 



Pollination and growth of the pollen tube. Upon the dehiscence of the 

 anther and shedding of the pollen from the pollen sacs, some pollen 

 grains either fall upon or are carried by some means to the stigma of 

 the same or of another flower. The transfer of pollen from the anther to 

 the stigma is called pollination. This process may be brought about by 

 wind, water, insects, man, and gravity. On the stigma the pollen grain 

 germinates almost immediately, and a pollen tube develops from it. The 

 stigma is covered with a sticky fluid of sugars, acids, and other sub- 

 stances, and the pollen grain will usuallv germinate best on stigmas of 

 the same or closely related plants. Pollen grains of many kinds of plants 

 germinate when placed in solutions of sugar and water. 



The pollen tube grows rapidly, penetrates the stigma, and extends 



^ In the embryo sacs of some plants, such as triUium, the number of nuclei is four. One 

 becomes the egg, two form the fusion nucleus, and the fourth disintegrates. Several other 

 deviations in embryo sac development in other plants are known, but only the usual type 

 of embryo sac will be described here. 



