WINTER KILLING AND HARDINESS 257 



the lower total water content of hardy tissues and their greater content of 

 adsorbed water which is not readily frozen. Furthermore, the water- 

 holding capacity operates less effectively in dilute solutions. ^'^'* 



Water in the adsorbed or colloidal condition cannot hold materials in 

 solution but may cause higher results in sap density determinations. 

 Since in hardy plants there is a smaller amount of free water which can 

 hold materials in solution, the sap solutes must be held ordinarily in a 

 more concentrated solution than in tender plants. Hence the correlation 

 between sap density and hardiness found b}' Chandler and confirmed by 

 other investigators; however, since there is no direct causative relation 

 between hardiness and sap density, it sometimes happens that the correla- 

 tion does not hold. Pantanelli,^" for example, was unable to find a 

 relation between resistance to cold and molecular concentration (sap 

 densitjO with wheat or beets, though the correlation held for sunflower, 

 tomato and corn. 



Clear distinction is necessary between cell water loss and tissue 

 water loss. Cell water loss is the cause of death by freezing. Tissue 

 water loss must, in many cases, accentuate cell water loss and thus 

 indirectly lead to killing. Though the two forms are distinct, in most 

 cases each promotes the other. It is probable that some plants are reten- 

 tive of cell water and not of tissue water, hence hardy but not drought 

 resistant; others are presumably retentive of tissue water but not of cell 

 water, hence drought resistant but not hardy. However, there is a 

 strong tendency toward parallelism in drought resistance and cold resis- 

 tance. The data presented above are based on this parallelism. 



Xerophytic adaptations are well known to students of morphology; 

 they serve primarily as protection against tissue loss, but may have an 

 ultimate bearing on cell loss and therefore on hardiness. Strausbaugh'^^ 

 shows that the lenticel area on the twigs of a semihardy plum is from 

 three to six times that of a hardy variety. He shows also a greater loss 

 of water from the twigs of a tender than from those of a hardy variety. 

 Thus, occasionally, morphological differences may influence, though 

 indirectly, cell water loss. Rather extensive investigations, however, 

 have failed to establish consistent morphological differences between 

 hardy and tender varieties. Cell water loss, then, must depend on 

 something other than structure. 



Water-retaining Capacity Associated with Pentosan Content. — 

 Plant tissue which withstands freezing must be supposed to contain or be 

 able to manufacture substances which will hold water in an adsorbed or 

 colloidal condition. These substances must themselves be colloids, they 

 must have a great water-holding and water-absorbing capacity, they must 

 be known to occur in practically all plants capable of withstanding winter 

 conditions and they must be distributed generally through practically all 

 plant tissues. The compounds which answer best to these specifications 



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