WINTER KILLING AND HARDINESS 



259 



data, reported in Table 15, show the tomato to have a higher total 

 pentosan content than the hardier cabbage, but the soluble pentosans 

 are quite differently arranged. The increase of total pentosans in 

 cabbage of differing degrees of hardiness is due entirely to the increase 

 in soluble pentosans; with the tomato, in which no treatment materially 

 increases hardiness, there is no increase in soluble pentosans. 



Investigation is likely to show that other substances may exercise 

 water-retaining properties and therefore tend toward hardiness. Fat 

 emulsions conceivably may act in this way. 



Whatever the compounds may prove to be, their water-adsorbing 

 power undoubtedly is affected to a marked degree by the factors which 

 generally increase or decrease the water-retaining properties of colloids. 

 The effects of nitrogenous compounds and hydrogen-ion concentration 

 on hardiness have been emphasized by Harvey and may be of a similar 

 nature. 



Pentosan Content, Water-retaining Capacity and Hardiness Respon- 

 sive to Environmental Conditions.^ — The water-retaining capacity of 

 plant tissues is increased by any condition which limits the water sup- 

 ply without producing actual injury. This is discussed in the section 

 on Water Relations. Rosa presents data, given in Table 16, showing 

 an increase in the pentosan content of plants hardened by exposure to 

 low temperatures in a cold frame. This indicates also that the hardening 

 or maturing of plant tissue by exposure to cold is essentially a reaction to 

 a limited water supply. 



Table 16. — Pentosans of Vegetable Plants in Percentages of Fresh Weight 



{After Rosa^^^) 



Cabbage 



Leaf lettuce Cauliflower 



Moisture, series on greenhouse plants 



1. Tender plants grown in wet soil 



2. Medium hard}' plants grown with moder- 



ate moisture supply 



3. Hardy plants grown in dry soil 



4. Hardy plants, partly wilted for 2 weeks . . . 



Coldjrame series 



1. Tender greenhouse plants 



2. Hardened 1 week 



3. Hardened 2 weeks 



4. Hardened 3 weeks 



0.191 

 0.403 



Rosa shows also that there is a fairly constant increase in the pentosan 

 content of vegetables from early fall until the plants are killed by cold. 

 The data in Table 17 indicate that maturity is associated with increased 

 pentosan content and greater water-retaining capacity. 



