iqiq] MacDOUGAL, RICHARDS, &- SPOEHR— SUCCULENCE 415 



one. Two other features remain to be considered, that of the 

 composition of the plasmatic colloids, and of the salts dissolved in 

 the water of hydration. The colloids of living leaves are highly 

 kydrated, and the salts, acids, etc., are also in a highly dilute 

 condition, in which case their effect would be at a minimum. Death 

 and desiccation would be accompanied by a concentration of these 

 compounds, until finally they would be adsorbed by the cell walls 

 and plasmatic colloids in their most concentrated condition with 

 resulting coagulations, some of which in all probability are irre- 

 versible. The thin leaves have a higher acid content, and, to 

 anticipate, a smaller proportion of pentosans which would accen- 

 tuate this effect, hence the relatively low coefficient of swelling from 

 a dried state. A long series of experiments with sections of dried 

 colloids and of living and dried plants of known composition make 

 it appear that the water relations of active tissues show the behavior 

 of a biocolloid consisting largely of pentosans, of which agar or 

 plant mucilages would be an example, a small proportion of protein 

 or protein derivatives, and some salts and free acids.'' It is to these 

 features, therefore, that one would naturally turn for the factors 

 which might increase the water-holding capacity of the cell or 

 organ, and in so doing the pentosans would claim attention first. 

 These substances probably are always present in some proportion 

 in cells, and their occurrence is therefore not significant. Any 

 action or condition which brings about a notable increase in their 

 proportion in the cell would have most important consequences 

 however. Such increase does result from a depletion of the water 

 of a cell, for the polysaccharides under such conditions are reduced 

 to the pentosans, and the reduction of the water content of a cell 

 results in the conversion of the polysaccharides, which do not show 

 marked imbibition, to pentosans, which take the form of an elastic 

 gel with an enormous capacity for expansion, particularly when 

 mLxed with nitrogenous material, and upon this rests the hyper- 

 trophies or hyperplasias of thin- walled tracts in the development of 



'' MacDougal, D. T., Imbibitional swelling of plants and colloidal mixtures. 

 Science 44:502. 1916. 



MacDougal D. T., and Spoehr, H. A., The effect of acids and salts on biocolloids. 

 Science 45:269-272. 1917. 



, Growth and imbibition. Proc. Amer. Phil. Soc. 56:289-352. 1917. 



, The behavior of certain gels useful in the interpretation of the action of 



plants. Science 45:484-488. 1917. 



