DIV. ii PHYSIOLOGY 313 



apparently acts only in determining the assimilation of C0 2 , in the 

 second phase it has another significance a certain period of illumina- 

 tion is quite indispensable for this, and only the rays of greater wave- 

 length are effective, those of short wave-length even destroying the 

 state reached in the first phase. In nature the first phase is attained 

 in autumn, but a sufficiently long and intensive illumination is wanting. 

 Under continuous illumination by an Osram lamp, the light from which 

 is rich in red rays, the formation of flowers may be hastened by 

 months ; the earlier in winter this is done the longer is the illumina- 

 tion required, and the period is shortened by increasing the intensity 

 of the illumination. Interruptions in the illumination must not be 

 too prolonged or the influence of the illuminated period is lost. The 

 third phase of elongation is, like the first, dependent on the nutritive 

 effect of light ; in accordance with this, if the preceding nutrition has 

 been sufficient it may, in part at least, be carried out in the dark. 



Similar thorough analyses of the conditions of flowering are not available as 

 yet for other cases, but numerous observations and experiments indicate that 

 light, temperature, and the nutrient salts are of primary importance in the forma- 

 tion of the flowers. Since these factors are also indispensable for the vegetative 

 life of the plant, it is the amount in which they are available and especially their 

 relative proportions which determine whether a particular bud shall form a flower 

 or grow vegetatively. 



The importance of light in the formation of flowers is shown by the well-known 

 fact that the Ivy only flowers when growing in a well-illuminated situation and 

 not in the shade of woods, although it grows well in the latter habitat. VOECHTING'S 

 experiments on Mimulus Tilingii gave the same result. At a certain low 

 intensity of light, which is quite adequate for vegetative -growth, this plant 

 produces no flowers. KLEBS has made corresponding experiments with Veronica 

 Chamaedrys, 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- 

 formation. 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 plants of our climate eventually become vegetative in the 

 tropics (Cherry), 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 during the winter. In this way KLEBS succeeded in keeping 

 the Beet in a purely vegetative state for several years. Glechoma and Sempervi-vum 

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

 same way has been pointed out by BENECKE, who showed both from the literature 



