268 LECTURE XIII. 



upon the formation of any other waste-products. It seems to 

 be a well-established fact that light is of importance in the 

 formation of alkaloids, for tropical plants which produce these 

 substances in abundance in their normal habitat produce only 

 small quantities when grown in hot-houses in this country. 

 On the other hand it appears that too intense light is un- 

 favourable to the accumulation of alkaloids for it has been 

 observed in certain cases that plants which have grown in the 

 shade are richer in alkaloids than others which have been 

 exposed to the full glare of the tropical sun. This is probably 

 due to some decomposition of the alkaloids under the influence 

 of intense light. Similarly light appears to be unfavourable 

 to the accumulation of organic acids in the plant, for both 

 Wiesner and de Vries have observed that etiolated plants are 

 richer in organic acids than plants of the same kind which 

 have been grown in light. This is in harmony with the facts 

 mentioned in a previous lecture with reference to Mayer's 

 observations on succulent plants (p. 232), from which it ap- 

 pears that light causes the decomposition of organic acids 

 in the plant. Further, Rauwenhoff found that the cells of 

 an etiolated Polygonum cuspidatum contained no crystals of 

 calcium oxalate whereas they are abundant in green plants, 

 and that the amount of tannin in etiolated leaves and branches 

 of Polygonum Bistorta, Rosa centifolia, and Vicia Faba 

 was much smaller than in similar organs which had been 

 developed in the light. 



Finally, it appears probable that light also influences the 

 metabolism of plants by affecting the action of unorganised 

 ferments. Niepce de St Victor and Corvisart found that 

 starch was converted into sugar more rapidly in light than in 

 darkness, and Mayer observed that whereas the action of 

 iiwertin and of pepsin was unaffected by light, the action of 

 rennet (chymosin) was distinctly retarded. It is however im- 

 possible at present to form any estimate of the importance of 

 these facts in the metabolism of plants. 



Heat. It is a well-established fact that the temperature 

 of the surrounding medium has an important influence upon 



