THE DISPERSION OF SEEDS AXD SPOKES. 71 



of fertilisation, though disseminating the spores. 

 The case is entirely different, however, when, as 

 is frequently the case even in Ferns, the prothalli 

 are dicecious; and still more so in plants which, 

 like Isoltes and Selaginella, produce two kinds of 

 spores. The locomotive power of the antherozoid, 

 being of short duration, would hardly be sufficient 

 to carry it from one prothallus to another if there 

 was any considerable distance to be traversed. 

 There must, therefore, be what is equ.ivalent to a 

 separate sowing for each kind of spore. And, even if 

 cross-fertilisation be as important among Crypto- 

 gams as it is among flowering - plants, the only 

 means of accomplishing it is the mingling of spores 

 brought about by the wind. This mixing of male 

 and female spores practically amounts to wind- 

 fertilisation. Where a plant produces two kinds 

 of spores, the macrospores are fewer in number 

 than the microspores, and, consequently^ will be 

 more thinly sown. If the number of macrospores 

 be very small, the probability in any particular 

 instance that a microspore will alight upon one of 

 them is slight. Were there, then, no further pro- 

 vision than exists in Coniferae, fertilisation, and 

 especially cross-fertilisation, would rarely, if ever, 

 occur. But after the development of the male 

 prothallus every microsi^ore becomes a new^ centre 

 of dispersion from ^^•hich antherozoids are distri- 

 buted in every direction, so that the chances of fer- 

 tilisation, and even of crossing, are greatly increased. 

 We therefore see that in the heterosporous Cr^^p- 

 togams, and in those producing male and female 

 prothalli, the process of fertilisation may be regarded 

 as consisting of two parts. The first part of the 

 process, viz., the distribution of the micro- (or male) 

 spores, is effected by the wind ; the second, or delivery 

 of the antherozoids to the archegonium, is the 

 work of these locomotive bodies themselves. The 

 swarming movement of the antherozoids more 

 than compensates for the absence of an expanded 



