§1] 



UPON THE RATE OF GROWTH 



42a 



night. Not all seedlings and fungi are equally affected by 

 light ; the effect depends upon their normal conditions of illu- 

 mination — the conditions to which they have become attuned. 

 2. Accelerating Effect of Light. — Although certain species 

 of phanerogams are little affected in their growth by light, 

 actual acceleration probably rarely occurs in this group. The 

 parasitic mistletoe (Viscum album) seems to form an impor- 

 tant exception, however, since, according to Wiesnek ('79, p. 

 183 ; '98, p. 506), it neither grows nor germinates in the dark. 

 This fact is correlated with a peculiarity in its phototropic 

 response, as we shall see later (p. 438). Among aquatic algse 

 several cases of acceleration of growth by light have been 



^ Fig. 121. 



recorded. The flat, circular, green thallus of Coleochsete scu- 

 tata, when obliquely illuminated, grows, according to Kny 

 ('84), faster on the side next to the light. Again, Spirogyra 

 which has been kept in the dark until all of its starch has been 

 consumed grows when placed in the light, but does not grow 

 in darkness (Famixtzin, '67). Lately Whipplb ('96) has 

 given quantitative data on the relation between intensity of 

 light and of growth in diatoms. A known quantity of dia- 

 toms of one or two species was placed in a bottle of water, 



