282 BOTANY part i 



Chamacdrys, and he states that in all plants which do not contain any great amount 

 of reserve materials a diminution of light leads to the suppression of flower- 

 foi'mation. He regards the carbon-assimilation resulting from the illumination as 

 the primary cause of this influence on the development of flowers. At a certain 

 intensity of light, which is insufficient for the development of normal flowers, 

 cleistogamous flowers are produced. 



Temperature also obviously plays a part. A continuous high temperature 

 hinders flowering. Thus plantsof our climate eventually become vegetative in the 

 tropics (Cherry, cf. p. 272), and native perennial plants, such as the Beet or Foxglove, 

 can be prevented from flowering in their second year if they are kept warm and 

 allowed to grow on duiing the winter. In this way Klebs succeeded in keeping 

 the Beet in a purely vegetative state for several years. GlecJioiiw, and Sempervivuvi 

 also, if their winter rest is prevented, grow vegetatively for years. 



Lastly, the nutrient salts have to be considered. By removing the supply of 

 salts, seedlings can often be converted into dwarf starved plants in which, after a few 

 minute foliage leaves have been formed, the development of flowers begins at once. 

 Experiments of Moebius have shown that Grasses and Borago flower better if the 

 supply of salts is limited than if well manured. The increase of fertility 

 which results from root-pruning in fruit trees may depend upon a limitation of 

 the absorption of niitrient salts. That, however, all nutrient salts do not act in the 

 same way has been pointed out by Benecke, who showed both from tlie literature 

 and from his own experiments that nitrogenous food led to a diminution and 

 phospihorous to an increase in the development of flowers. 



D. 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. Tiie Garlic, which forms small bulbils in place of flowei's, 

 the White Lily, and Ranuncuhis 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. No degeneration 

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

 multiplication is to be observed in these cases. 



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 rejDroduction. 



Were the Algae and Fungi alone taken into consideration it might 

 be supposed that sexual reproduction led to the formation of specially 

 resistant germs wliich could endure a longer period of rest under 

 unfavourable conditions — as a matter of fact the zygospores and 

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



