146 PLANT PHYSIOLOGY 



of starch. In the ordinary Irish potato, when the tubers are ex- 

 posed to temperatures as low as 7° C, the starch is transformed 

 into sugar. As much as 3% of sugar may be formed, but when the 

 temperature is raised the sugar which is not respired is turned 

 back into starch. In maple trees the food is stored in the form 

 of sugar, which is rapidly converted into starch in the spring upon 

 the approach of warm weather. Lidforss, by feeding sugars to 

 hothouse plants, succeeded in making them resistant to —7° C. 

 in some cases. 



While the presence of sugar seems to be associated with low 

 temperatures and assists the plant to keep a high osmotic pressure 

 so that the proteins are not precipitated by freezing (Chap. XIX), 

 some plants, like the sugar beet, are rich in sugar and nevertheless 

 rather sensitive to cold weather. However, cool weather facilitates 

 the transfer of sugar to the root, which explains in part the favor- 

 able effect of cool autumns upon the percentage of sugar in beet 

 roots. Although the sugar beet is not very hardy, it is doubtless 

 hardier than it would be without the sugar. 



QUESTIONS 



1. What are the two large classes of carbon compounds? How do they 

 differ? 



2. Discuss the construction of the polarimeter. 



3. What is reduction? 



4. What is the characteristic grouping (radical) in an organic acid? 



5. How many kinds of isomers are found in the sugars? 



6. Why is a knowledge of the chemistry of the carbohydrates necessary 

 for a student of plant physiology? 



7. Why is the maple sugar season so short? 



8. Why do potatoes sometimes get sweet in cold weather? 



9. How do pentosans help to make the plant resistant to low temperature? 

 to dry weather? 



10. Are all sugars equally usable by the plant? (See Knudson, 1926.) 



REFERENCES 



Appleman, C. O., and Arthur, J. — Carbohydrate metabolism in green sweet 



corn during storage at different temperatures. J. Ag. Res., 17:137, 1919. 

 Blackman, F. — The biochemistry of carbohydrate production in the higher 



plants from the point of view of systematic relationship. New Phyt., 



20:2, 1921. 

 Blood, P. — Carbohydrate reserves of young apple trees as influenced by 



winter storage conditions. Proc. Am. Soc. Hort. Sci., 19:33, 1922. 



