INFLUENCE OF EXTERNAL STIMULI. LIGHT 



243 



might lead us to the conclusion that formation of flower is independent 

 of light, for we see bulbs of hyacinth, tulip, and other plants, bring- 

 ing forth their flowers 

 in darkness, and even 

 on completely etiolated 

 seedlings of Phaseolus 

 vulgaris, Vicia Faba, and 

 Cucurbita Pepo we can 

 occasionally prove the 

 beginning of the forma- 

 tion of flower 1 . In the 

 first set of these cases 

 however we have simply 

 to do with the unfolding 

 of the flower-buds al- 

 ready laid down in the 

 bulbs, and in the second 

 we are dealing with 

 seeds which are rich 

 in reserve-materials the 

 most favourable for for- 

 mation of flower. If we 

 place in darkness plants 

 of, say, Brassica, Tropae- 

 olum, Papaver, Cucur- 

 bita, or others, already 

 provided with flower- 

 buds, these buds will not 

 unfold if the plants have 

 been withdrawn from the 

 light in too early youth ; 

 older buds unfold them- 

 selves but often less com- 

 pletely, and in Tropae- 

 olum there appears a 

 kind of cleistogamy in- 

 asmuch as some of the 

 flowers which do not 

 unfold show rudiments 

 of seeds. On the other 

 hand the etiolated plants 



FIG. 121. Campanula rotundifolia. Shoot with linear leaves which 

 was placed in conditions of feeble illumination. The flower-buds, K, 

 which were already laid down have been arrested. A lateral shoot, A. 

 has developed, bearing rounded leaves ; such shoots are never produced 

 under normal conditions. If flower-buds had not been laid down the 

 chief shoot itself might have produced rounded leaves after the linear 



1 Sachs, Gesammelte Abhandlungen, i. p. 207. 



R a 



