182 MANUAL OF BOTANY 



The Gametophyte. — As in the Oymnosperms we have two 

 forms of the gametophyte, the one commg from the microspore, 

 the other from the macrospore. 



The prothaUimn developed from the microspore is more 

 reduced than in the last group. When the spore is shed, or 

 sometimes while it is still in the sporangium, the nucleus divides 

 into two, but this division is not followed by the formation of a 

 cell-wall. The protoplasm of the spore aggregates round the two 

 nuclei, forming two jJ^^imordial or naked cells, one of which is 

 the generative, the other the vegetative cell. The exine ruptures, 

 often at particular spots, but sometimes irregularly, and the 

 intme grows out into a long tube, the pollen tube. In some 

 cases, as in the Malvaceae, several tubes arise fi^om different 

 pomts on the spore. 



The pollen tube is derived from the larger of the two cells, 

 the vegetative one, as in the Gymnosperms. The generative cell 

 passes into the tube, as does the vegetative nucleus. As they 

 proceed down the tube, the generative cell divides into two, 

 forming two gametes, which correspond to the antherozoids of 

 lower forms, but the}' are never differentiated as m the latter 

 case. The vegetative nucleus becomes disorganised and disap- 

 pears. 



By the time that the pollen tube has reached the gametophyte 

 of the macrospore the process is complete, and the tube remains 

 with its two gametes at or near its apex. The protoplasm of the 

 tube is richly vacuolated, and the granules it contains show ex- 

 tensive movements of circulation. 



The macrospore produces its gametophyte in its interior as 

 in the Gymnosperms, but the prothallar tissue is verj^ much 

 smaller (fig. 935). The nucleus of the macrospore divides into 

 two, and each travels to one end of the spore. Each then divides 

 into two, and each of the daughter nuclei repeats the process, so 

 that at that stage the spore contains two groups each of four 

 nuclei, one at the micropjdar and the other at the chalazal end. 

 One nucleus from each of these groups then travels to the centre 

 of the spore, and the two fuse together, constituting what is known 

 as the definitive nucleus of the embryo sac. A certain amount 

 of protoplasm aggregates round each of the three nuclei at the 

 two ends, forming two groups of three cells each. Those at the 

 chalazal end secrete ceU-walls and form the antipodal cells, 

 which take no further part in the development ; those at the 

 micropylar end remain naked and constitute the egg apjp)aratus. 

 One of these, whose nucleus is the sister nucleus of the one 



