THE GYMNOSPERMAE : CONIFERALES AND TAXALES 68i 



thin-walled side downwards. This side, from which the pollen tube develops, 

 is thus brought into contact with the surface of the nucellus. 



The Male Gametophyte. 



Before the pollen is shed the first nuclear division has occurred, and 

 immediately after pollination a second division of the nucleus of the tube 

 cell produces a second prothallial cell on top of the first, equally flattened 

 and evanescent (Fig. 682). 



The remaining nucleus now divides a third time, cutting ofl^ a large, 

 rounded cell on top of the degenerating prothallial cells. This is the 

 antheridial cell, so called because it produces the male generative cells, 

 though it forms nothing resembling an antheridium in structure. The 

 nucleus which remains in the tube cell of the microspore apparently controls 

 the germination of the spore and is called the tube nucleus (Fig. 683). 



When the microspore has made contact with the tip of the nucellus the 

 exo-intine breaks between the air sacs and the intine grows out to form the 

 beginning of the pollen tube, into which the tube nucleus passes. The 

 pollen tube penetrates into the nucellar tissue and grows slowly throughout 

 the ensuing summer. [Meanwhile the scales of the female cone thicken till 

 the cone is once more completely closed, thus shutting in the developing 

 pollen, which rests throughout the first winter. 



The following April the pollen tube begins to grow again. The antheridial 

 cell divides into two cells, one of which we may call the body cell and the 

 other the stalk cell. The latter may represent the stalk of an ancestral 

 antheridium, while the body cell is a vestige of the remainder of the antheri- 

 dium. The stalk cell functions simply as a support for the body cell and 

 develops no further. The body cell divides into two unequal cells with 

 scanty cytoplasm and very large nuclei. If we were dealing with a fully 

 organized antheridium we would say these were antherozoid mother cells, 

 but they actually function as male gametes without developing into anthero- 

 zoids, so it is perhaps simplest to call them the male cells. 



The Female Gametophyte 



During the period which succeeds pollination the female cone increases 

 greatly in size, it turns green, and both the axis and the ovuliferous scales 

 become very much enlarged. By the end of the first year it is about 4 to 

 5 cm. long (Fig. 684). 



Meanwhile the development of the ovule continues. The megaspore 

 enlarges and its nucleus divides some eleven times in succession, giving about 

 two thousand nuclei, which are arranged in a layer of protoplasm around the 

 periphery of the embryo sac, surrounding a large central vacuole. These are 

 all " free " nuclei with, at first, no intervening walls. New walls then begin 

 to appear, growing inwards from the megaspore or embryo sac wall, and these 

 walls grow centripetally, like open tubes, drawing the cytoplasm with them, 

 until they meet at what was previously the middle of the vacuole. The embrv^o 



