424 EFFECT OF LIGHT [Ch. XVn 



covered with bolting cloth, and suspended in the water of a 

 reservoir, at the surface and at various depths below the sur- 

 face down to 20 feet. Experiments were made at a time 

 when the temperature of the reservoir water was almost the 

 same at all depths. After several days the bottles were col- 

 lected and the number of diatoms in each determined. The 

 results of one series of experiments, shown graphically in Fig. 

 121, demonstrated that the growth of diatoms is directly pro- 

 portional to the intensity of light received by them.* We 

 may conclude that in general, excepting perhaps the mistle- 

 toe, those plants whose growth is accelerated by light normally 

 have their growing parts fully exposed to sunlight. Their pro- 

 toplasm is attuned to a high intensity of light — is not retarded 

 by it ; indeed, demands it for the normal exercise of its func- 

 tions. 



Passing now to the subject of germination, we find that the 

 first development of the spores of the higher cryptogams usu- 

 ally requires or is favored by light (HoFFMANisr, '60, p. 321, 

 and others). The spores of many ferns, of the moss Poly- 

 trichum commune (BoEODiisr, '68), of the hepatics Duvalia and 

 Preissia (Lbitgbb, '77), and of Vaucheria do not germinate at 

 all in the dark. However, this result is not universal among 

 the higher green cryptogams, for MiLDE ('52, p. 627) observed 

 that spores of Equisetum germinated in the dark as well as in 

 the light ; and it is clear that certain fern spores (Ophioglos- 

 sum) can do so, for they germinate when covered by soil to a 

 depth of 3 to 5 cm. It is somewhat unexpected to find 

 the spores of such dark-lovers as ferns and hepatics nor- 

 mally dependent for their germination upon light. One calls 

 to mind, however, the fact that it is the habit of such spores to 

 germinate on the surface, where their prothalli are found — 

 hence at such times always subjected at least to a diffuse light. 



Certain seeds, also, are said to germinate more readily in the 

 light than in the dark. We have already cited the case of the 

 mistletoe ; the same seems to be true of the meadow grass, 

 Poa, as is shown by the following experiments of Stebler 

 ('81). Two species were experimented with, and two experi- 



* However, Whipple found that growth at the surface, where full sunlight 

 fell upon the bottle, was less than at a few Inches below the surface. 



