THE METABOLISM OF PLANTS. 271 



dioxide with evolution of oxygen will not go on below a 

 certain degree of temperature. Cloez and Gratiolet observed 

 that it began in Potamogeton between 10 and I5C, and in 

 Vallisneria above 6 C., an observation which has been con- 

 firmed by Sachs. Boussingault detected an evolution of 

 oxygen from leaves of the Larch at o'5 2*5 C., and from 

 those of meadow grasses at 1-5 3*5 C. : in the case of 

 Holtonia pahtstris Heinrich found that bubbles of oxygen 

 were first given off at 27 C. We may just repeat here that 

 the absorbent activity of the roots and the activity of trans- 

 piration depend very much upon temperature (see Lectures 

 IV. p. 52, and VII. p. 108). 



We will now consider the modes in which exposure to 

 cold causes the death of plants. It has been found, in the 

 first place, that long continued exposure to a not very low 

 temperature proves fatal. This fact has been ascertained by 

 experiments made with seeds. It has been already pointed 

 out (p. 172) that if seeds be kept for a long time they lose 

 their vitality, and it appears that the same effect is produced 

 by exposing them for a comparatively short time to a low 

 temperature. 



Haberlandt kept seeds of a number of plants for four months in a 

 vessel surrounded by melting ice, and then sowed them at a temperature 

 of 1 6 C. He found that seeds of the following plants had germinated to 

 a certain extent during the four months : 



Out of 300 Rye seeds nearly all had germinated. 



213 Hemp 30 



1 50 Vetch 3 



100 Pea 6 



245 Mustard 30 



305 Red Clover 29 



361 Lucerne 138 



200 Gold-of- Pleasure I 



a. Very slight elongation of radicle ; Rye, Hemp, Vetch, Pea. 



b. Considerable elongation of radicle ; Mustard, Red Clover, Lu- 



cerne, Gold-of- Pleasure, 



and that seeds of the following had not germinated at all : 



Wheat, Barley, Oat, Rye-grass, Buckwheat, Beet, Rape, Poppy, 

 P'lax, White Clover, Bean. 



