Factors in Evolution 39 



darkness, will do so if the temperature is raised to 

 32. It has also been demonstrated that the char- 

 acter of the nutrition may affect the power of the 

 plant to endure higher temperatures. Thus Theile 

 found that the common blue mold (Penicillium) 

 would cease to grow at a temperature of 31 when 

 cultivated in a sugar solution, but would endure 

 a temperature of from 35 to 36 when fed with 

 formic acid and glycerine. 



Endurance of Cold by Seeds. There seems to 

 be no degree of cold which is sufficient to kill dor- 

 mant protoplasm. Perfectly dry seeds of various 

 kinds have been exposed to the temperature of liquid 

 hydrogen ( 200), without affecting the power of 

 germination later on when exposed to suitable con- 

 ditions. 



Relation to Water. We have already seen that 

 the activity of protoplasm is dependent upon an ade- 

 quate water supply, and therefore the presence of 

 water is essential for the growth of all plants. The 

 water supply is perhaps the most powerful agency 

 of all in determining the form of the plant and its 

 organs. In its normal condition the cell is strongly 

 distended, and tissues composed of such turgid cells 

 are firm and elastic. With the withdrawal of a 

 portion of the water the tissue becomes flaccid 

 " wilts," as the gardener says of the plant which 

 droops for lack of sufficient water. To maintain 

 the tissues in their normal turgid condition, the plant 

 must provide for a loss of water due to evapora- 



