376 GYMNOSPERMS LESS. 



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

 of its protoplasm, becomes filled with small cells representing 

 a prothallus (prtJi). 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. sf). 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. 

 Protoplasm collects round each of the daughter nuclei, con- 

 verting them into cells, one of which remains undivided, 

 while the other divides, and its substance passes from the 

 pollen-tube into the ovum, where it forms a cell-like body, 

 to which the name of male pronucleus (see p. 263) has 

 been applied. This conjugates with the nucleus of the 

 ovum, or female pronucleus, and thus effects the process 



