732 GENERAL CONDITIONS OF PLANT-LIFE. 



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 reiains its vitality if thawed slowly 

 after the freezing of the water of its cell-sap, becomes disorganised 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, two 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 micellar interstices of the cell-wall and of the protoplasm. 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^. When 

 therefore a portion of the cell-sap-water freezes, the remainder of the cell-sap becomes 

 more concentrated ; and chemical changes may possibly be induced, as RiidorfF has 

 shown, by new combinations actually arising in a freezing solution. How far this 

 circumstance m.ust be considered in the destruction of cells by freezing and thawing is 

 not yet decided. 



What takes place in the freezing of a saturated organised body capable of swelling up 

 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 water freezes ; the rest 

 remains as water of imbibition between the micellae 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 coagulated 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. 



When living succulent tissue freezes, a portion only of the 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 

 and succulent stems frozen at a temperature between - 5° and - 10° C. it is easily seen 

 that only a portion of the water is present in the form of crystals of ice ; another portion 

 permeates the cell-walls which are not rigid but .still flexible. If the congelation takes 

 place slowly, the water assumes on the surface of the succulent tissue the form of a 

 coating of ice consisting of densely crowded small crystals. These crystals stand at 

 right angles to the surface of the tissue, and increase by growth at their base. A 

 very large portion of the water of a tissue may in this way take the form of a coating 

 of ice, while the tissue, becoming less watery, contracts in proportion^, and loses its 



^ 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 Landwirthschaftliche Versuchsstationen, i860, Heft. V. 

 p. 167, and in my Handbook of Experimental Physiology. I do not find that Goppert's objections 

 (Bot. Zeitg,, 1871, no. 24) affect my results; to his experiment on Calanthe veratrifolia quite a 

 different explanation can be given from that suggested by him. 



2 Riidorff, Pogg. Ann. 1861, vol. CXIV. p. 63 ; and 1862, vol. CXVI. p. 55. 



3 When this contraction operates unequally on different sides of a leaf or branch, it is easy to see 

 that curvatures must result which are indeed actually frequently observed. The splitting of the 

 trunks of trees in consequence of frost is probably only the result of changes of this nature. 



