476 UNTOWARD INFLUENCES. 



everywhere known as Warclian cases, the plants are supplied 

 with sufficient water, and the atmosphere is practically satu- 

 rated with moisture. When exposed to sunlight, the plants in 

 the cases can carry on all the operations of assimilation, growth, 

 and respiration. 



Comparing the conditions which surround the plants in a 

 Wardian case with those which prevail in a furnace-heated house, 

 it is plain that the plants in the case are placed in what is es- 

 sentially a humid tropical climate, while those in the house 

 are exposed to excessive dryness, and to an atmosphere which 

 ma}' contain minute traces of the poisonous gases arising from 

 combustion. 



1242. Liquids and Solids. Comparatively few substances 

 except those possessing strong acid or alkaline properties are 

 injurious to a plant. As indicated in 685, preparations of arsenic 

 which are extensively employed for the destruction of insects 

 upon crops in cultivated fields are not absorbed by plants to an 

 appreciable extent. This is further illustrated by the impunity 

 with which various other insecticides can be applied to green- 

 house plants. 



1243. Numerous experiments, more curious than profitable, 

 have been made to test the effect of poisonous alkaloids upon 

 vegetation. Many observers have proved that some plants 

 yielding poisonous alkaloids may be poisoned by applications 

 to their roots of solutions of the very alkaloids which they have 

 themselves produced ; thus morphia may poison the popp} r 

 (see 961). Strasburger 1 says that morphia speedily kills motile 

 spores. 



Kiihne 2 has noted that the protoplasmic movement in the 

 stamen-hairs of Tradescantia is not wholly arrested, even after 

 man} T hours, by a solution of veratrin ; and Pfeffer 3 has observed 

 that the cells in sections of certain fleshy roots are not killed 

 even when immersed for several days in a saturated solution of 

 morphia acetate. 



As Frank 4 suggests, these discrepancies in effects depend on 

 the differences in the power possessed by the various parts in the 

 absorption of such matters. 



1244. Effects of mechanical injuries upon the plant. The most 

 important of these are caused by destructive fungi. The destruc- 



1 Wirkung des Lichtes und der Wcarme auf Schwarmsporen, 1878, p. 66. 



2 Untersnchungen liber das Protoplasma, 1864, p. 100. 

 8 Pflanzenphysiologie, ii., 1881, p. 454. 



4 Pflanzenkrankheiten, 1879. 



