io86 A TEXTBOOK OF THEORETICAL BOTANY 



can be little harm in using the common parlance, which is often con- 

 venient. 



The pollen grain consists of a single cell with two wall-layers; the 

 extine or exine which is thickly cuticularized and protective, and the 

 intine which is very thin. The interior of the cell is densely filled with 

 cytoplasm and often contains abundant food reserves in the form of minute 

 starch grains or oil-drops. When mature it contains two nuclei; one of 

 which is the nucleus of the pollen grain itself, as a cell, while the other is 

 enclosed within a delicate oval or lens-shaped membrane, forming a cell 

 within a cell. This is the generative nucleus, from which are produced the 

 male gametes. There is no trace of vegetative prothallial cells such as are 

 found in most Gymnosperms, the pollen grain nucleus is the only vestige of 

 the male prothallus. 



The first stage in the sexual process is the transference of the pollen 

 from the opened anther to the stigma, a process known as pollination. 

 It may be brought about by movements of the parts'within a single flower 

 (self-pollination) but is more often carried out by external agencies, either 

 by the wind (anemophily) or by insect visitors (entomophily) who carry 

 the pollen from flower to flower (cross-pollination). 



The ovules are borne upon a cushion of specialized tissue within the 

 ovary, called the placenta. Each ovule consists of a central mass of tissue, 

 the nucellus, which is usually surrounded by two coats or integuments 

 and is attached to the placenta by a stalk or funicle (Fig. 1056). The 

 integuments do not completely enclose the nucellus, but leave, at one end, a 

 small opening, the micropyle, through which fertilization normally takes 

 place. The opposite end or base of the ovule is called the chalaza. 



Within the nucellus is the embryo sac, which is often so enlarged as to 

 occupy most of the nucellus. At maturity it commonly contains eight nuclei ; 

 three at the micropylar end, which constitute the egg apparatus, one of them 

 being the oosphere and the other two the synergidae; two nuclei He in 

 the middle of the sac, and are known as the polar or endosperm nuclei; 

 while three more nuclei form a group at the other end of the sac and are 

 called the antipodal nuclei. When a pollen grain has been transferred to 

 the stigma it absorbs water and its contents begin to swell; the extine cracks 

 and the intine protrudes from the opening as a thin-walled sac. Food re- 

 serves in the grain are hydrolysed, vacuoles are formed and growth begins. 

 The protruding sac extends and very rapidly develops into a long outgrowth, 

 the pollen tube, into which pass the contents of the grain. The pollen grain 

 nucleus goes first and now becomes the tube nucleus, and the tube itself, 

 penetrating the tissues of the style, grows towards the ovary and eventually 

 reaches the placenta, from which it emerges and passes across any inter- 

 vening space to the micropyle of an ovule, being guided apparently by chemo- 

 tropic attraction (see Volume III). As the tube approaches the ovule, the 

 generative nucleus divides into two and forms two vermiform nuclei, the 

 male gametes. The tip of the tube penetrates the nucellus and on making 

 contact with the embryo sac the walls of both structures are dissolved, 



