244 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



form vegetative shoot-parts, whose mass should be sufficient for the 

 formation of new flowers if it was merely a question of mass of formative 

 substance and not of its special nature. Sachs assumed the existence of a 

 special ' flower-forming material ' which is produced in the leaves and only 

 under the influence of light. This can however be transferred to parts 

 of plants placed in the dark, as is proved in nature by the development 

 of flower-buds on subterranean bulbs and tubers, and experimentally 

 by leading a portion of a shoot into a dark chamber whilst a number of 

 the leaves remain in light on the new shoots formed in darkness numerous 

 flowers arise ; eventually abnormalities appear, and these may be accounted 

 for by the fact that the path of the flower-forming material from the 

 leaves is always getting longer. 



Sachs l was led by subsequent experiments with Tropaeolum to the 

 conclusion that the ultra violet rays are specially needed for formation 

 of flower. Plants were cultivated behind a screen of a solution of sulphate 

 of quinine and only in exceptional circumstances did the formation of 

 a single flower take place ; usually the formation of flower was entirely 

 suppressed. The plants themselves, it may be remarked, were only 

 slightly etiolated but otherwise normal. The question of the significance 

 of the ultra violet rays requires however more extended investigation 2 . 

 It is certain that the intensity of light plays an important part in the 

 formation of flower, and that for this a much higher intensity of light 

 is required than suffices for the formation of vegetative organs. To 

 this conclusion the researches of Vochting 3 led him. He says : ' In order 

 that plants may form flower in a normal way the illumination must not 

 sink below a certain amount which is very unequal in different species. 

 Shade-plants and sun-plants require different degrees of illumination 

 for the performance of the same functions. . . . Impatiens parviflora, for 

 example, a shade-plant, produces complete flowers under illumination 

 which would scarcely enable Malva vulgaris, a sun-plant, to form buds. . . . 

 If the illumination is allowed to sink below the required amount, the size 

 of the whole flower, or of its individual parts, is diminished and with 

 decreasing illumination a stage is reached at which formation of flower 

 entirely ceases. In many species the stage at which complete cessation 



1 Sachs, tJber die \Yirkung der ultravioletten Strahlen auf die Bliitenbildung. Gesamrnelte Abliand- 

 liuigen, p. 293. 



3 C. de Candolle has published in ' Etude de 1'action des rayons ultra- violets sur la formation des 

 fleurs,' in Archives des Sciences phys. et natur. xxviii (1892), the result of a repetition of Sachs' 

 experiments. He found no flowers in two plants after cultivation behind a screen of solution of 

 sulphate of quinine for seventy-one days ; thirty-three flower-buds in two plants grown behind an 

 equally thick screen of water ; behind a screen of aesculin flowers were formed in Lobelia Erinus, but 

 in smaller numbers than behind water. 



3 Vochting, Uber den Eiufluss des Lichtes auf die Gestaltung und Anlage der Bluten, in Pringsh. 

 Jahrb. xxv. p. 149. 



