Lecture XXIV. 207 



is immediately underneath the micropyle. Early in the second 

 year of the cone the endosperm is thus an egg-shaped mass occupy- 

 ing the greater part of the ovule. About this time certain cells on 

 the surface of the endosperm grow in size. These cells are about five 

 or six in number and are situated round the pole of the endosperm 

 which is nearest to the micropyle. These each divide into an 

 inner and outer cell : from the former an ovum is developed and 

 the latter produces the short neck of an archegonium. Evidently 

 the endosperm is a gametophyte and the cell in which it develops 

 is a megaspore quite comparable with that of Selaginella. The 

 ovum grows rapidly in size, and the adjoining cells of the gameto- 

 phyte differentiate as a capsule or venter enclosing the ovum, and 

 convey to it the necessary materials for its growth. The cells sur- 

 rounding the archegonium grow up round the neck, so in this way 

 the latter finds itself at the bottom of a slight depression. The 

 ovum, which by this time has grown so as to be about i mm. in 

 length and is consequently easily visible to the naked eye, contains 

 a large nucleus which is located in its upper end near the neck. 

 This nucleus now divides just as the nucleus of the ovum of the 

 archegoniates divides in preparation for fertilisation. In this 

 division a small lenticular mass of protoplasm is separated im- 

 mediately below the neck. It contains one segment of the 

 nucleus and forms the ventral canal-cell, while the other segment 

 persists as the nucleus of the mature ovum which now awaits 

 fertilisation. 



To understand how fertilisation is effected we must return to the 

 pollen which has been shed from the pollen-cone. This dust is 

 scattered abroad in great quantities so that some of it falls on, or 

 is wafted to, the ends of all the branches in the neighbourhood of 

 the pollen-cones. Some falls on the young seed-cones standing 

 erect at the top of the current season's growth. The carpels of 

 these are divergent and some of the pollen-grains find their way 

 down between them to their base. Here they are detained by a 

 drop of sticky fluid exuded from the micropyles of the immature 

 ovules. As this fluid decreases by evaporation or absorption the 

 pollen-grains are drawn by its retreating surface through the 

 micropyle and are left adhering to the surface of the nucellus. 

 The tube-cell of the pollen-grain now bulges out and presses into 

 the top of the nucellus, firmly anchoring the grain there. The 

 carpels of the cone close the spaces between each other by their 

 growth and thus the pollen-grains are completely shut in from 

 adverse conditions and remain quiescent during the autumn, 

 winter and spring. 



