154 On Recent Researches in Vegetable Physiology. 



very different proportions. But a multitude of plants occur 

 under very different conditions, and perish as soon as their 

 cells have felt the attacks of frost. At what precise moment 

 do they die? M. Goeppert* cites in connexion with this an 

 observation (an isolated one, it is true, but still curious) which 

 seems to prove that it is the direct action of cold, tlie frost it- 

 self, that kills delicate plants. Two tropical Orchidefe, Phajus 

 grandifolius and Calanthe veratrifolia^ contain considerable 

 quantities of indigo in their flowers. This substance, as every 

 one knows, is colourless in living plants, and only becomes 

 blue after their death, by a phenomenon of oxidation. The 

 flowers of these two plants are of a fine white colour ; but it 

 is only necessary to rub them a little hard with the hand to 

 bring out in them the natural tint of indigo. Cold produces 

 exactly the same effect : as soon as the flowers are frozen, no 

 matter to what extent, their corollas immediately become deep 

 blue ; and this colour persists after thawing. In this case, at 

 least, the cells have been killed by the direct action of cold. 



One of the most characteristic features of the cells which 

 have suffered from frost is the modification of their endosmotic 

 properties : they lose their turgescence, and the liquid which 

 they contain escapes through their walls without the least 

 effort. M. Sachs has sought the explanation of these facts in 

 the modifications which the molecular structure of the mem- 

 brane suffers under the influence of thaw. If this comes on 

 suddenly, the shock destroys the existing molecular equi- 

 librium. This would be an effect similar to that which we 

 observe under analogous circumstances in white of egg or 

 starch-paste. After thawing, these two substances no longer 

 present any thing but a spongy mass without consistency, 

 allowing the liquid which they contained to escape under the 

 smallest pressure. 



M. Prilleuxf has opposed this opinion, which presupposes, 

 according to him, the formation, in the invisible pores of the 

 membrane, of icicles, which, by their fusion, would overthrow 

 the molecular equilibrium. Now the properties of capillary 

 spaces, and the difficulty of causing water to freeze in them, 

 are by no means favourable to this theory. The properties of 

 the frozen cells being exactly the same as those of cells which 

 have passed through boiling water, M. Prilleux proposes to 

 seek the explanation of the phenomenon in the- alteration of 

 the protoplasm, and not of the membrane. It is, in fact, upon 

 the diosmotic properties of the primordial utricle that the se- 

 paration of the different liquids enclosed in the different cells 



* Botan. Zeit. 1870, No. 24. 



t Bull. Soc. Botan. do France, 1869, xvi. p. 91. 



