246 THE EFFECT OF TEMPERATURE 



light were only 10 to 13 mm. high. Again, he found 

 that the hypocotyl of the buckwheat (Fagopyrum) 

 reached a height of 35 to 40 cm. in the dark, whilst it 

 grew only to 2 or 3 cm. when freely exposed to light. 

 K. Goebel* has shown that if cactuses are cultivated in 

 darkness, their form changes completely. The young 

 shoots are rounded, and fail to show the angular irregu- 

 larities of form which increase the surface capable of 

 effecting assimilation under the influence of light. 



Darkness conduces to increased growth, therefore, or 

 conversely, light tends to retard growth. That this is 

 the case is well shown by an observation of Wiesner.f 

 This observer exposed seedlings of the vetch (Vicia 

 sativa) under a glass globe to sunlight for YJ hours. 

 When placed horizontally, so as to get the full force of 

 the sun's rays, no growth whatever occurred, but when 

 placed vertically, so that the growing part of the seed- 

 ling was more or less protected by its leaves, there was 

 an increase in height of about .8 mm. On the other 

 hand, a control seedling kept in a darkened globe grew 

 about 2.8 mm. in the same period. This retarding effect 

 of light is not universal, however. It is practically ab- 

 sent in some cases, as of the yam and of a wild gourd 

 (Bryonia), and in those plants whose rapidly growing 

 parts are sheltered from the sun's rays by protecting 

 coverings it is but little evident. Still Sachs' conclu- 

 sion as to the effect of daylight on growth probably ap- 

 plies with greater or less force to the majority of plants. 

 Thus he found J that during the night the growth 



* Flora, Ixxxi. p. 96. 



f Davenport's "Experimental Morphology," p. 41. 



\ Arb. aus der Bot., Inst. Wiirzburg, i. p. 99. 



