6;'54 GENERAL CONDITIONS OF PLANT-LIFE. 



small quantity of water, stand the cold of winter and frequent rapid thawing; while 

 the young leaves at the time of their unfolding in the spring succumb to a slight night- 

 frost. An at least equally important condition lies however in the specific organisation 

 of the plant ; varieties of the same species frequently diifering in their power of resist- 

 ance to cold and thawing. Some plants, like Mosses, Hepaticae, Lichens, some Fungi 

 of a leathery texture, the mistletoe, &c., appear in particular never to freeze ; Pfitzer 

 states that the Naviculeae freeze between — 12° and — 25° C. and continue to live after 

 thawing ; while many flowering plants from a southern climate are killed by rapid 

 changes of temperature near the freezing-point \ 



Whether the tissue of a plant can be killed simply by the solidifying of the water 

 contained in its cells into crystals of ice is uncertain ; while on the other hand it is un- 

 questionable that in a great number of plants death is caused only by the mode in which 

 the thawing takes place. The same tissue which retains its vitality if thawed slowly 

 after the freezing of the water of its cell-sap, becomes decomposed if thawed rapidly 

 after exposure to the same degree of cold. Death is therefore caused in these plants 

 not by the freezing but by the thawing ^. 



When ice is formed in the tissues of a plant, tw^o points must be taken into con- 

 sideration. The water, when about to freeze, is on the one hand contained in a mixed 

 solution, the cell-sap ; on the other hand it is retained by the force of cohesion as water 

 of imbibition in the molecular pores of the cell-w all and of the protoplasmic bodies. 

 Now it is an established fact in physics that a solution when freezing separates 

 into pure water which solidifies into ice and a concentrated solution with a lower 

 freezing-point^. A portion of the cell-sap-w^ater becomes therefore by freezing more 

 concentrated than the part which is not frozen ; and chemical changes may possibly be 

 induced, as Riidorff has shown, by new^ combinations actually arising in a freezing 

 solution. How far this circumstance must be considered in the destruction of cells by 

 freezing and thawing is not yet decided. 



W^hat takes place in the freezing of a moistened and woollen organised body is 

 somewhat similar to that which occurs in a freezing solution. In this case also, when 

 the temperature falls to a certain point, only a portion of the imbibed water freezes ; 

 the rest remains as water of imbibition between the molecules of the body, which 

 contracts, while the freezing portion of the water of imbibition separates to form ice- 

 crystals. This phenomenon happens in a striking manner in starch-paste; a homogeneous 

 mass before freezing, it has the appearance after thawing of a spongy coarsely porous 

 structure, the water running off clear from its large cavities. The behaviour of coa- 

 gulated albumen on thawing is exactly the same. In these cases a permanent change 

 has clearly been brought about by the freezing of a portion of the imbibed water ; the 

 molecules of the substance which group themselves into a network containing but 

 little water when ice is formed in paste or coagulated albumen, on thawing no longer 

 combine with the portions of the water which separated from them on freezing into 

 a homogeneous whole ; the thawed paste is in fact no longer paste. 



Even when living succulent tissue freezes, a portion of the imbibed water separates 

 and freezes as pure water, the rest remaining as water of imbibition in the protoplasm 

 and the cell-walls, at least as long as the temperature does not sink very low. In leaves 



^ On the maximum of temperature which vegetation can in general bear see Giippert, Bot. Zeitg., 

 1871, nos. 4 and 5. 



2 The correctness of this statement is supported by a careful series of observations which I 

 communicated to the konigl. sachs. Gesellsch. der Wissensch, i860, On the formation of crystals, &c., 

 and which will be found also in the Landwirthschaftlische Versuchsstationen i860, Heft V, p. 167, 

 and in my Handbook of Experimental Physiology. I do not find that Goppert's objections (Bot. 

 Zeitg., 1 87 1, no. 24) affect my results; to his experiment on Calmithe veratrifolia quite a different 

 explanation can be given from that suggested by him. 



2 Riidorff, Pogg. Ann 1861, vol. CXIV, p. 62,; and 1862, vol. CXVI, p. 55. 



