CONDITIONS OF REPRODUCTION 459 



life if sufficient light, which is essential to the formation of 

 flower rudiments, is available, as is the case in spring. Such 

 a mature plant placed in a richly manured hotbed in March 

 is prevented from flowering in the following summer, though 

 the same treatment in April is without eff"ect. In April 

 flowering can be prevented by cultivation in the dark or 

 in blue light. By the first method the absorption of nitrates 

 is accelerated, by the second assimilation is depressed, 

 while in both dissimilation predominates over assimilation. 

 Klebs has shown that in the plants prevented from flowering 

 the soluble nitrogen compounds are more and the soluble 

 carbohydrates less abundant than in flowering plants of the 

 same age. His experiments make it probable that the 

 nutritive conditions determine whether the plant shall 

 develop reproductively or vegetatively. We know little 

 of the intimate reactions concerned ; we do not know what 

 nitrogen compounds are involved, nor do we know in any 

 case what the critical ratio is. Other authors, e.g. Sachs, 

 have postulated the formation of special flower-forming 

 compounds ; there is no experimental evidence for this 

 theory, which does not also support the clearer hypothesis 

 of Klebs. 



§ 12. Senescence, Death, and Individuality 



The death of the annual, of the biennial, and of some 

 perennials is closely associated with the achievement of 

 reproduction ; the plant produces seed and then dies. 

 The connection of the one event with the other may be 

 emphasised by experiment. The mignonette is an annual, 

 but, by carefully removing the flower buds, it may be made 

 to persist through several years, growing into a little shrub 

 (Molisch, 1921). The turnip is a biennial, but, by keeping 

 it over winter in a hothouse, it may be forced to vegetate 

 for two or three years, evidently because the high rate of 

 respiration and growth maintained depletes the food store 

 to a certain extent. 



In perennials there is rarely this relation, and their life 



