Factors Affecting Growth and Reproduction 213 



Recent investigations indicate that much of importance may 

 be learned from a more careful study of the effect of soil tempera- 

 tures upon plant form and behavior. Very low soil temperatures 

 have been reported to shorten the vegetative period of beets, with 

 the result that only slender roots are obtained, while very high 

 temperatures shorten the vegetative period of kohl-rabi and pre- 

 vent the formation of the thickened stems for which the plant is 

 cultivated. Wheat germinated at low temxperature produces its 

 adventitious root system at a favorable depth beneath the soil 

 surface. If germinated at higher temperature the adventitious 

 root system is produced at or above the surface of the soil, with 

 the result that a very weak plant results even if the conditions 

 after germination are the best obtainable. 



Disease-producing organisms are much more destructive at 

 one soil temperature than at another. If the best temperature 

 for the growth of the organisms differs from that of the host 

 plant, the disease may often be avoided by planting when the 

 soil temperatures are favorable to the crop and unfavorable to 

 the disease-producing organism. 



Freezing. When plant tissues freeze, the formation of ice 

 takes place in the intercellular spaces. As the ice forms, water 

 is withdrawn from the cells, just as it is when a plant wilts. The 

 result of this withdrawal of water is a greater concentration of 

 the salts inside the cell and a higher osmotic pressure. Water- 

 imbibing substances and the osmotic pressure resist the outward 

 movement of water. In general it has been found that those 

 plants and tissues that have the highest water-holding power 

 are also most resistant to freezing injuries. They are likewise 

 least affected by drought. One of the sources of injury to the cell 

 when a large proportion of its water is removed is the precipitation 

 of proteins. Young growing tissues usually have a high water 

 content and the cells contain but Kttle of the water-holding sub- 

 stances ; consequently they are very susceptible to freezing in- 

 juries. 



