Chap, hi.] VEGETAL ASSIMILATION AND DIS-ASSIMILATION. 119 



in a glacier, and send down numerous roots, several inches in 

 length, into ice destitute of crevices.^ It is then probable that 

 germination can still take place at degree, and, as it always 

 develops a remarkable quantity of heat, the result must have 

 been, in the case mentioned by M. IJloth, the partial fusion of 

 the ice, whence it became possible for the roots to penetrate. 



We have given the limits of temperature, but these vary 

 according to species. Most plants succumb after remaining for 

 ten minutes in water at from 45 to 46 degrees. In the air, 

 phanerogams are killed after remaining from ten to thirty 

 minutes in a temperature of from 50 to 51 degrees. To produce 

 a lasting alteration, it is sometimes necessary that the tempe- 

 rature should reach 60 degrees. De^th then takes place through 

 the coagulation of the albuminoidal substances. 



Dried vegetal tissues (seeds) can naturally support more 

 extreme temperatures of heat or cold. Thus, tissues full of 

 sap are killed at 50 degrees, whilst dried seeds of Pisum sativum 

 may be heated with impunity to 70 degrees for an hour. 



In every plant each function has its special limits of tempe- 

 rature. We cannot enter here into the enumeration of facts 

 in detaU. Moreover^ we have already stated the temperatures 

 necessary for the operation of chlorophyll. 



In cells killed either by congelation or by too high a tempe- 

 rature, the cellular membranes become permeable ; liquids filter 

 through : consequently turgescence ceases. First of all, the pro- 

 toplasm becomes immovable, takes a sombre tint, and rapidly 

 loses its water by passive evaporation. 



Life is not sufficiently active in plants for them to have, like 

 the higher animals, a temperature of their own, independent in 

 some degree of that of the exterior medium. The small aquatic 

 plants, and the subterraneous parts of terrestrial plants, have 

 generally the temperature 'of the ambient medium. On account 

 of their volume, the massive stems follow more slowly the 



' Uloth, Flora, 1871. 



