298 Introduction to Botany. 



the same place in the life cycle of a flowering plant. The 

 pollen spore consists of a single cell (Fig. 159, d\ as does 

 the spore of a moss or fern. We find, however, that after 

 a time, and even before it is discharged from the anther, 

 the pollen spore has become two-celled (e). The germina- 

 tion of the spore of a moss or fern began by the division 

 of its single cell, one cell becoming two, two becoming 

 four, etc., until the complete gametophyte is formed ; and 

 we may conclude from this that the division of the pollen 

 spore is the beginning of its germination and of the 

 formation of the complete gametophyte. Following the 

 further behavior of the pollen, as described on page 166, 

 we find that after it has been transferred to the stigma its 

 inner wall forms a tube penetrating to the ovule, and that 

 one of its two cells passes through the tube to the ovule, 

 having first divided, or subsequently dividing, to form two 

 cells, one of which fuses with and fertilizes the egg cell, 

 and for that reason must be considered a sperm. From 

 this we are led to the conclusion that the three cells result- 

 ing from the germination of the pollen spore constitute a 

 gametophyte. The great elongation of the inner wall of 

 the pollen spore producing the pollen tube does not have 

 its counterpart in the germinating spores of ferns and 

 mosses. The pollen tube functions in part as an anther- 

 idium, although being apparently not its homologue. 



The behavior of the embryo sac spore will now be 

 traced. It is a single large cell (m) which never becomes 

 discharged from the place of its formation within the 

 ovule. Usually before the descent of the pollen tube its 

 nucleus divides and the daughter nuclei continue the 

 process of division until eight nuclei have been formed, 

 four of them taking position at the micropylar and four at 

 the opposite end of the embryo sac (n). One from each 



