DIV. ii PHYSIOLOGY 315 



cases, however, "barren " fruits develop wholly without the stimulus of pollination 

 (parthenocarpic ( 76 ) fruits of the Fig, Cucumber, and certain species of Apple and 

 Pear). 



Influences which affect parts at a distance also proceed from the pollen-grains and 

 pollen-tubes on the stigma. Thus after the stigma of an orchid is pollinated the 

 stigma and the gynostemium swell, and the perianth is promptly arrested in its 

 growth and withers. As FITTING ( 77 ) showed, this influence proceeds from soluble 

 organic substances which withstand heating, and can be readily separated from the 

 mass of ungerminated pollen. 



Whether a simple spore or a complex embryo is the result of 

 fertilisation it is always distinguished from the cells which gave 

 rise to it by exhibiting nuclei which contain the diploid number of 

 chromosomes (p. 203). On this account a reduction division which 

 restores the normal number of chromosomes is sooner or later the 

 necessary sequel to fertilisation. 



2. The Significance of Sexual Reproduction 



The significance of sexual reproduction is not at once evident. 

 Many plants occur in nature or under cultivation without being 

 sexually reproduced, and succeed with vegetative reproduction only. 



Lower plants which have not attained to sexual reproduction have already been 

 referred to. Of higher plants which no longer produce descendants sexually the 

 cultivated Bananas, some Dioscoreaceae, some forms of Vine, Oranges, and Straw- 

 berry may be mentioned. The Garlic, which forms small bulbils in place of flowers, 

 the White Lily, and Ranunculus Ficaria, which has root-tubers, only rarely produce 

 fertile seeds if allowed to form their vegetative organs of reproduction. Under 

 certain conditions, as for instance on cut inflorescences, seeds may be produced, 

 though as a rule these plants are multiplied entirely vegetatively. ISTo degeneration 

 such as was formerly held to be unavoidably associated with purely vegetative 

 multiplication is to be observed in these cases ( 78 ). 



If thus the monogenic reproduction suffices to maintain the species 

 digenic reproduction must serve some further purpose not effected by 

 the former. Otherwise it would be inconceivable why digenic repro- 

 duction had arisen, and why the arrangements to effect it are far more 

 complicated and less certain than in the case of vegetative reproduction. 



Were the Algae and Fungi alone taken into consideration it might 

 be supposed that sexual reproduction led to the formation of specially 

 resistant germs which could endure a longer period of rest under 

 unfavourable conditions as a matter of fact the zygospores and 

 oospores are much more resistant than the swarm-spores and conidia. 

 But even in the Pteridophyta this relation is inverted, for the fertilised 

 egg-cell requires to develop forthwith, or else it perishes, while the 

 asexual spores can endure a long resting period. 



It is the rule in digenic reproduction that the sexual cells are 

 individually incapable of development; this takes place only after the 

 sexual cells have united. Thus one use of fertilisation lies in the 



