INTRODUCTORY DISCUSSION. 7 



idea of the relative stability of these substances in simple solution. It is 

 now becoming more evident that under conditions such as exist in an 

 organism (i. e., in the presence of various catalysts such as inorganic salts, 

 enzyme, acids, and alkalies), the molecules of proteins, fats, and especially 

 carbohydrates break apart almost spontaneously. From a vast amount of 

 physiological evidence now at hand, it appears that the energy of an 

 organism is derived chiefly from the break-down of the material ingested 

 as food. When the supply of nutritive material is insufficient the nature 

 of the respiration changes and then, in all probability, the more stable 

 substratum is drawn upon. This becomes evident in the cacti as well as in 

 many other plants. 



Kosinski 1 has shown that when Aspergillus niger is grown on pure water, 

 the carbon dioxid given off falls rapidly to a value about one-quarter of 

 that when grown on sugar. During this time the organism is probably 

 drawing on the more stable protoplasmic substances. When sugar is made 

 available, the carbon dioxid rises immediately, indicating the direct utiliza- 

 tion of that substance. 



The colloidal materials found in plants are in general relatively chemi- 

 cally stable substances, although their exact chemical nature and mode of 

 formation as yet have not been definitely established. It now seems prob- 

 able that in a sense they represent by-products or end-products of the 

 metabolic process. The nature of this material undoubtedly varies in 

 different plants, especially in regard to the proportion of carbohydrate and 

 proteinaceous substances. This is indicated in the study of the behavior 

 of the colloids of various plants. Extensive comparative investigations with 

 different vegetable tissues, prepared " biocolloids," and especially with the 

 cacti, have indicated that these plants behave like masses of gels composed 

 largely of colloidal carbohydrates. 2 



In the present investigation an effort has been made to determine the 

 nature of the carbohydrates of the cacti and to study the transformations 

 which these undergo under various conditions. In the conception of the 

 colloidal nature of protoplasm briefly outlined, it has been stated that this 

 is of necessity a relatively stable system and that the colloidal material is 

 itself a product of metabolism. One of the most striking and interesting 

 features of these plants is the presence of large quantities of pentosans and 

 mucilaginous material. In the cacti it has been found that water imbibition, 

 swelling, and growth are intimately related to the presence of pentose 

 polysaccharides. The pentosans have been found as components in varying 



1 KOSINSKI, I. Die Atmung bei Hungerzustanden und unter Einwirkung von 



mechanischen und chemischen Reitzmitteln bei Aspergillus niger. Jahrb. 



wiss. Bot., 37, 137-204, 1898. 

 'MAcDouGAL, D. T., and H. A. SPOEHR. The behavior of certain gels useful in the 



interpretation of the action of plants. Science, n. s., 45, 484-488, 1917. 



, . Growth and imbibition. Proc. Amer. Phil. Soc., 44, 289, 1917. 



MAcDouoAL, D. T. Imbibitional swelling of plants and colloidal mixtures. Science, 



44, 502-505, 1916. 

 McGEE, J. M. The imbibitional swelling of marine algae. The Plant World, 21, 



13-15, 1918. 



