IRRITABILITY. 91 



the other hand, if the soil is too wet there is a tendency for the 

 roots to grow away from the soil which is saturated with water. 

 In such cases roots are often seen growing upon the surface of 

 the soil so that they may obtain oxygen, which is important for 

 the root in the processes of absorption and growth. Plants then 

 may be injured by an excess of water as well as by a lack of 

 water in the soil. 



214. Temperature In the experiments which have thus far been carried 



on it will probably have been noted that the temperature has much to do 

 with the length of time taken for seeds to germinate. It also influences the 

 rate of growth. The effect of different temperatures on the germination of 

 seed can be very well noted by attempting to germinate some in rooms at 

 various temperatures. It will be found, other conditions being equal, that 

 in a moderately warm room, or even in one quite warm, 25-30 degrees cen- 

 tigrade, germination and growth goes on more rapidly than in a cool room, 

 and here more rapidly than in one which is decidedly cold. In the case of 

 most plants in temperate climates, growth may go on at a temperature but 

 little above freezing, but few will thrive at this temperature. 



215. If we place dry peas or beans in a temperature of about 70° C. for 15 

 minutes they will not be killed, but if they have been thoroughly soaked in 

 water and then placed at this temperature they will be killed, or even at a 

 somewhat lower temperature. The same seeds in the dry condition will 

 withstand a temperature of 10° C. below, but if they are first soaked in water 

 this low temperature will kill them. 



216. In order to see the effect of freezing we may thoroughly freeze a sec- 

 tion of a beet root, and after thawing it out place it in water. The water is 

 colored by the cell-sap which escapes from the cells, just as we have seen it 

 does as a result of a high temperature, while a section of an unfrozen beet 

 placed in water will not color it if it was previously washed. 



If the slice of the beet is placed at about 6o° C. in a shallow glass vessel, 

 and covered, ice will be formed over the surface. If we examine it with the 

 microscope ice crystals will be seen formed on the outside, and these will 

 not be colored. The water for the formation of the crystals came from the 

 cell-sap, but the concentrated solutions in the sap were not withdrawn by 

 the freezing over the surface. 



217. If too much water is not withdrawn from the cells of many plants in 

 freezing, and they are thawed out slowly, the water which was withdrawn 

 from the cells will be absorbed again and the plant will not be killed. But 

 if the plant is thawed out quickly the water will not be absorbed, but will 

 remain on the surface and evaporate. Some will also remain in the inter- 

 cellular spaces, and the plant will die- Some plants, however, no matter how 



