518 MANUAL OF BOTANY 



attached to the parent sporophyte, such a means of fertilisation is 

 of course impossible. For fertilisation to occur it is necessary 

 that the two gametophytes shall be produced in close pro- 

 pinquity to each other. This is effected by the bringing 

 together of the two spores concerned in developing them. 

 The microspore or pollen grain is carried by various means to 

 the neighbourhood of the macrospore ; in the Gymnosperms 

 it falls upon the macrosporangium itself ; in the Angiosperms 

 upon the stigma of the pistil in which the macrosporangia are 

 hidden. As it germinates, its gametophyte, the pollen-tube, 

 makes its way through the intervening tissues till it reaches the 

 macrospore itself, close to the archegonium in the first case, and 

 to the oosphere when there is no archegonium. Fusion of the 

 gametes produced in each gametophyte then becomes possible, 

 and by a deliquescence of the separating walls they come into 

 contact with each other and their union takes place. 



In these higher plants we have two processes necessary for 

 sexual reproduction, the one the approximation of the spores, 

 or pollinatio7i, the other the fusion of the gametes, ov fertilisa- 

 tion. Frequently the latter term is loosely and erroneously 

 applied to the former process, as when the fertilisation of 

 flowers is spoken of. The process is one of pollination, and 

 may be followed by fertilisation or not according to circum- 

 stances. 



When the cells which fuse are derived from the same game- 

 tophyte, the term self -fertilisation is applied to the process ; 

 when they come from different ones it is called cross-fertilisa- 

 tion. Self-fertilisation in the strict sense is extremely rare ; 

 even in the Thallophytes the uniting cells usually originate 

 on different plants. The term ' self- fertilisation ' has been used 

 very generally to indicate the pollination of a pistil by pollen 

 from the same flower ; it is however better to speak of this as 

 self-pollination. Speaking strictly, self-fertilisation cannot take 

 place in heterosporous plants, as each spore produces its 

 appropriate gametophyte and the two sexual cells never arise 

 upon the same one. Cross-fertilisation has been found always 

 to result in stronger and healthier plants than self-fertilisation. 

 Cross-pollination, or the bringing together of spores from 

 different flowers, or from different plants, of the same species, 

 also yields more and better seeds than self-pollination. 



Very many mechanisms have been developed in different 

 plants to secure this end. Pollen may be carried from flower 

 to flower by wind or water, or by the agency of insects or other 



