372 GYMNOSPERMS 



rounded by a wall or integument (/) also formed of a small- 

 celled tissue. The integument is in close contact with the 

 nucellus, but is perforated distally by an aperture, the 

 micropyle (mpy\ through which a small area of the nucellus 

 is exposed. 



Each megasporangium contains only a single megaspore 

 (embryo sac, c and D, mg. sp) in the form of a large ovoidal 

 body embedded in the tissue of the nucellus. It has at 

 first the characters of a single cell, but afterwards, by division 

 of its protoplasm, becomes filled with small cells representing 

 a prothallus (prt/i). As in Vascular Cryptogams, single 

 superficial cells of the prothallus are converted into ovaries 

 which are extremely simple in structure, each consisting of 

 a large ovum (ov\ and of a variable number of neck-cells. 



The pollen, liberated by the rupture of the microsporangia, 

 is carried to considerable distances by the wind, some of it 

 falling on the female cones of the same or another tree. In 

 this way single microspores (pollen-grains) find their way 

 into the micropyle of a megasporangium (D, mi. sp). This 

 is the process known as pollination, and is the necessary 

 antecedent of fertilization. 



The microspore now germinates : the outer coat bursts, 

 and the larger of the two cells (B, b) protrudes in the form 

 of a filament resembling a hypha of Mucor, and called a 

 pollen-tube (D, p.f). This forces its way into the tissue of 

 the nucellus, like a root making its way through the soil, 

 and finally reaches the megaspore in the immediate neigh- 

 bourhood of an ovary. A process then grows out from the 

 end of the tube, passes between the neck-cells, and comes in 

 contact with the ovum. 



In the meantime the nucleus of the large cell (b) of the 

 microspore that from which the pollen-tube grows has 

 travelled to the end of the pollen-tube and divided into two 



