BOOKS AND CURRENT LITERATURE 



Sugar Metabolism in Cacti. — In a recently published monograph 

 Spoehr 1 presents and discusses the results of some scores of analyses of 

 cacti subjected to various natural and experimental conditions. The 

 monograph is much more than a summary of analytical data. The 

 causes and results of the changes in carbohydrate content and in the 

 ratios of the individual carbohydrates to each other would follow the 

 seasons, the water-balance of the plant, the temperature, the external 

 food supply and the respiratory activity are discussed most suggestively 

 but with results which it is impossible to review in detail without 

 repetition of the comprehensive chemical data. 



The feature of the results which seems to the reviewer of greatest 

 present interest is the role of the pentose sugars, a matter already 

 noted by Spoehr in these columns. 2 These compounds, which are 

 differentiated from the ordinary or hexose sugars by having their car- 

 bon atoms arranged in groups of five instead of six, are represented in 

 plants mainly by the pentosans, — complex polysaccharids which are 

 the gums or mucilages present in many vegetable cells. These mucil- 

 lages are prominent in the cacti and Spoehr has shown that they may 

 be consumed, on occasion, in the metabolism of the plant. Further- 

 more, it appears that conditions of low water supply favor the forma- 

 tion of these pentosans in the cell, and this suggests a possible mechan- 

 ism for the production of large, mucilaginous cells, and therefore of 

 succulent habit, as a response to dry conditions. 3 The great capacity 

 of these gums for the imbibition of water, with the resultant swelling, 

 gives them important place in the colloidal changes which accompany 

 growth. 4 The known presence of pentose sugars in the nucleus of the 

 plant cell completes the picture of a group of carbohydrates probably 



1 Spoehr, H. A. The Carbohydrate Economy of Cacti. Carnegie Inst. Wash., 

 Pub. 287, 79 pp., 1919. 



2 The Plant World, 21:. 365-379 (1917). 



3 See MacDougal, Richards and Spoehr, "Basis of succulence in plants," — 

 Bot. Gaz. 67:405-416 (1919), and MacDougal and Spoehr, "The origination of 

 xerophytism,"— The Plant World 21:245-249 (1918). 



4 See MacDougal and Spoehr, "Growth and imbibition," — Proc. Amer. 

 Phil. Soc. 56:289-352 (1917). 



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