734 



GENERAL CONDITIONS OF PLANT-LIFE. 



melt off very quickly, a portion of the water runs into the interstices of the tissue 

 before it can be absorbed ; the original normal degree of concentration of the cell-sap 

 and degree of imbibition of the cell-wall and protoplasm cannot be re-established in the 

 cells; and this, depending upon the chemical nature of the substances dissolved in the 

 cell-sap and upon the conditions of the micellar structure of the protoplasm and of the 

 cell- wall, may be fatal. It is evident, on the view here taken, that the danger of freezing 

 increases with the amount of water in the tissue ; for the less watery the tissue the more 

 concentrated is the cell-sap and the larger is the proportion of water retained by the 

 force of imbibition ; only a small portion of the water can therefore form ice-crystals, 

 and when they thaw the injurious effects are not so great. 



We can now also understand why some plants are killed by being thawed too quickly 

 when they have been frozen by very severe cold, while freezing by a moderate amount 

 of cold is not injurious to them ; for the lower the temperature falls the larger is the 

 proportion of the cell-sap and water of imbibition that is converted into ice ; the dis- 

 turbance of the degree of concentration of the sap and of the imbibition of the cell- 

 wall is always greater with the increase of the cold ; and therefore the restoration of 

 the normal condition on thawing more difficult. That the splitting asunder of whole 

 masses of tissue during freezing such as has been described has but little effect on the 

 continuance of the life of the organ after thawing, is shown by the fact that even the 

 leaf-stalks of the Artichoke, the frozen state of which is represented in Fig. 473, remain 

 uninjured till the following summer if thawed slowly. These internal rupturings have 

 as little to do with the sudden destruction of the life of the cells from cold as the splitting 

 of the trunks of trees caused by frost, which, when the temperature falls very low, is 

 produced by the contraction of the bark and outer layers of wood, the crevices again 

 closing when the temperature rises. 



The idea that growing plants, especially those which require a high temperature for 

 their growth, can be directly killed by the cooling of their tissues for a short time nearly 

 to the freezing-point is shown by H. de Vries' experiments (/. c.) to be fallacious. The 

 older observations of Bierkander and Hardy that some plants of this description (e.g. 

 Cucurbitaceae, Impatiens, the Potato, Bixa Orellana, Crescentia Cujete, Sec.) freeze when 

 exposed to the air at low temperatures above the freezing-point, may nevertheless be 

 explained if it is recollected that the temperature of their tissues may fall below the 

 freezing-point from radiation, even when that of the air is 2° or 3° or even 5° C. above 

 it. But there is another way in which low temperatures above zero are injurious to 

 plants from southern climates, 'vt%. when the soil about the roots remains for a consider- 

 able time at this low temperature while the leaves continue to transpire. In this case 

 the absorption of water through the roots becomes so slow that they are no longer able 

 to replace the loss caused by evaporation from the leaves, which in consequence wither, 

 and at length altogether dry up. It is then sufficient to warm the soil about the roots, 

 in order to revive the withered leaves ; as I found in the case of plants of Nicotiana, 

 Cucurbita, and Pbaseolus grown in pots\ In England the branches of a Vine which 

 were made to grow into a hothouse, while the roots stood in the ground outside, withered 

 in winter, evidently only from the low temperature of the ground ; for when this was 

 watered with warm water, the branches in the hothouse recovered. 



3. Among the changes caused in plants by long-continued depression of temperature, 

 one of the most striking is the change in colour of leaves which persist through the 

 winter, originally observed by MohP, and recently more minutely studied by Kraus^. 



^ Sachs, in Landwirthschaftliche Versuchsstationen, 1865, Heft V. p, 195. 



2 Mohl, Vermisdite Schriften ; Tubingen, 1845, p. 375. 



3 Kraus, Observations on the winter colouring of evergreen plants; in the Sitzungsber. der 

 phys.-med. Societat zu Erlangen, Dec. 19, 1871, and March 11, 1872; also Bot. Zeitg.. 1874. 

 £Batalin has shown (Bot. Zeitg., 1874), and his observations have been confirmed by Askenasy 

 {Bot. Zeitg, 1875), that this change of colour is due rather to the influence of light than to that 

 of cold.] 



