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1920] 



MACDOUGA l—biocolloids 



127 



which furnishes many valuable homologies.- These are in imminent 

 danger of being overworked, however. Not only has gelatine been 

 taken to simulate protoplasm in general, but physicists have com- 

 mitted a similar error of taking the behavior of gelatine as universal 

 for colloids. 



Not much progress had been made in the attempt to determine 

 the physical basis of growth in plants before it became plainly 

 obvious that the relations of plant protoplasm to H ion concentra- 

 tion, acidity, temperature, and other conditions diverged widely or 

 were directly the reverse of the action of gelatine. 



In seeking other material in a colloidal state which might by 

 its hydration, swelling coefficient, etc., simulate the plant, recourse 

 was had to actual analyses of plants as made in connection with 

 studies in desiccation, starvation, etiolation, and as part of the 

 work on the carbohydrate metabolism of plants by Spoehr. 1 



The possible importance of the pentoses as a factor in the 

 mechanism of hydration and growth was evident at once. A 

 knowledge of the manner of formation and occurrence of these 

 sugars and of their condensation products, the pentosans (mucilages, 

 slimes, gums, etc.), is necessary in order to understand the role they 

 play in the cell. 



The metabolism of the plant is predominantly carbohydrate, 

 and the protoplasm may be taken to contain solutions of sugars 

 at all times. Included among the numerous possible changes, it is 

 known that in the depletion of the water content of a plasmatic 

 mass by a general loss from the cell, or by lowered hydration 

 capacity of the colloid by the action of any agent, polysaccharides 

 which have but little imbibition or hydration capacity are reduced 

 to pentosans or mucilages which have a relatively enormous 

 capacity for taking up water. Dextrose, starch, wall material, etc., 

 may be involved in these conversions, and, when masses of material 

 are affected, layers or globules of mucilage may be formed which 

 may react to microchemical tests. It is noted, however, that 

 visible masses of mucilage are of less importance in the mechanism 



1 Spoehr, H. A., Carbohydrate economy of the cacti. Publ. no. 287. Carnegie 

 Inst. Wash., p. 44. 



, The pentose sugars in plant metabolism. Plant World 20:365. 1918. 





