THE METABOLISM OF PLANTS. 275 



The effect of freezing is to make the organs hard and 

 brittle, and, in the case of succulent organs, to give them a 

 transparent glassy appearance. The destructive effect of 

 freezing upon succulent organs is very clearly exhibited when 

 they have been rapidly thawed ; they are quite flaccid, be- 

 cause the protoplasm of the cells is now no longer capable 

 of maintaining their turgidity (p. 40). The permeability 

 of the protoplasm is demonstrated by the fact that in cells 

 in which coloured cell-sap is present, the protoplasm becomes 

 stained by the colouring matters when the cells have been 

 killed by freezing (p. 44). 



From the various facts with which we have now become 

 acquainted we may draw some general conclusions as to the 

 relation of cold to the life of plants. We see, in the first 

 place, that the power of enduring extreme cold is possessed 

 in different degrees by one and the same organ in different 

 plants. Martins compares each plant to a thermometer, the 

 zero-point of which is the minimum-temperature at which its 

 life is possible. Secondly, we learn that the different meta- 

 bolic processes do not all stand in the same relation to tem- 

 perature ; that, in any given plant, some of the processes 

 can go on at lower temperatures than others, each process 

 having its own zero-point. Thirdly, that the larger the 

 proportion of water in an organ, the more liable it is to be 

 injured by frost : hence the zero-point for the life of any 

 given organ will vary according to the amount of water 

 which it contains at different times. 



We have learned, so far, that a certain temperature is 

 essential to the maintenance of the life of the plant, and we 

 have now to study the effect of different and relatively high 

 temperatures upon it. The relation of temperature to the 

 metabolic and other processes of plants may be generally 

 stated thus ; that, as the temperature rises above the zero- 

 point for any given process, that process is performed with 

 greater activity. Some illustrations of this have been already 

 given with reference to absorption (p. 52), transpiration (p. 

 1 08), respiration (p. 198) and germination ; to these the fol- 

 lowing may be added. Sachs found that whereas the roots 



1 8 2 



