184 PLANT GROWTH 



temperatures in this region it is usually because of lack of 

 moisture. 



Plants can be conditioned to endure low temperature. 

 It is called "hardening" and is brought about by decreasing 

 the water supply and subjecting them to progressively lower 

 temperatures alternating with higher ones. The tempera- 

 ture is often alternated at 36° and 50° F. then gradually 

 lowered. Cold injury is usually due to a lack of water in the 

 whole plant or in some of the cells of the plant. Transpira- 

 tion will go on at a reduced rate in cold weather while absorp- 

 tion is retarded considerably, especially if the ground is 

 frozen. When cells freeze the water begins to freeze and as 

 the water freezes more is withdrawn from the protoplasm. 

 This may continue until the protoplasm has its water re- 

 duced to the point where it is injured. Frequently slow 

 thawing enables the protoplasm to regain the water without 

 injury. Hardened plant cells have a higher colloid and sugar 

 content which resists the loss of water and may explain the 

 value of the hardening process. Plants that normally remain 

 in the soil during the winter should be kept cold to keep them 

 dormant, since they withstand cold better in a dormant con- 

 dition. Mulching should be done when the ground is frozen. 



Water constitutes from two-thirds to more than 95 per 

 cent of living, growing plants. Any deficiency of water first 

 reduces the turgor pressure of the cells and may in extreme 

 cases be so limited that the protoplasm will not be elabo- 

 rated. The elaboration of protoplasm and enlarging of the 

 cells, due to water absorption, are not synonymous but it is 

 difficult if not impossible to measure them independently. 

 The explanation for the rapid growth of plants with water 

 culture solution rests on the adequate supply of water at all 

 times. Methods of watering plants were discussed in Chap- 

 ter 13. 



The writer's interest in the water relation of plants led 

 to greenhouse experiments on the influence of soil moisture 

 on growth. Morning glory and moonvine plants were grown 



