458 GYMNOSPERMS LESS. 



frequently called the embryo sac (c and D, mg. sp), and 

 having 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 nucleus and 

 protoplasm, becomes filled with small cells representing a 

 prothallus (prtfi). As in Vascular Cryptogams, single super- 

 ficial 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, &n& is the necessary 

 antecedent of fertilisation. 



The microspore now germinates : the outer coat bursts, 

 and the vegetative cell (B, o) protrudes in the form of a 

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

 tube (D, p.t}. 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 neighbour- 

 hood 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 vegetative cell (b) 

 that from which the pollen-tube grows has travelled towards 

 the end of the pollen-tube and undergone degeneration. The 

 generative cell at the same time enters the pollen-tube and 

 divides into two sperm-cells. The end of the pollen-tube 

 becomes mucilaginous and one of the sperm-cells makes 

 its way through it, down the neck of the ovary and into the 

 ovum. The nucleus of the sperm-cell called the male 



