2 Hydration and Growth. 



with water. In the more liquid phase the molecules of the substance 

 are associated with such a large proportion of water as to be in a sus- 

 pended condition, while in the more solid phase the proportion of 

 water is much less. This phase has a distinct architecture which has 

 been likened to that of a mesh, felt, foam, or honeycomb, in which the 

 denser phase forms the framework and the fluid fills the interstices. 

 Under certain conditions the phases may be reversed, and the solid 

 particles be rounded into globules entirely surrounded by the fluid. 

 These structures are far too minute to be visible under the microscope, 

 as the particles which are dispersed in the liquid or are aggregated in the 

 denser structure ma each consist of a few molecules only. In addi- 

 tion to the material in the actual sponge of protoplasm, some of the 

 same or other substances may be present in dispersions or solutions in 

 cavities and spaces in the cell, which result from morphological or 

 mechanical action of the protoplast (see p. 21). 



The aggregation of molecules of a substance in a colloidal condition 

 such as that noted above is a more complex matter than that of the 

 solution of a crystalloidal compound, as, in addition to the forces of 

 chemical combination, surface tension results in adsorption or union 

 of substances in indefinite proportions. 



Four main groups of substances make up the protoplastic engine- 

 carbohydrates, proteins and their derivatives, the lipins, and the salts. 

 Perhaps all carbohydrates may exist in a colloidal condition, but the 

 group polysaccharids, including the pentosans, are the most important 

 in the architecture of the protoplasmic mesh, as these substances with 

 proteinaceous compounds appear to determine the water-relations of 

 living matter, and to contribute to the design of a machine in which 

 metabolism takes place. The proteinaceous substances may in plant 

 protoplasm form a widely varying proportion, generally very low, but 

 sometimes ranging as high as 90 per cent of the entire dry weight of 

 the protoplasm, with very important consequences as to imbibition. 

 The enzymes are included with these substances, and as the metab- 

 olism, including respiration of plants, is predominantly a complex of 

 transformations in carbohydrates, the possibilities of variation in this 

 feature are very great. The nature of the pentose derivatives present 

 in the protoplasm may also be a feature of considerable importance in 

 metabolism and water-relations, as suggested by the differential be- 

 havior of various gums and mucilages when compared with that of 

 agar. The colloidal carbohydrates, or those which enter into the 

 make-up or design of the living machine and the proteinaceous sub- 

 stances, are theoretically mutually nondiffusible, so that the gelation 

 or solidification of a 10 per cent solution of agar and gelatine or starch 

 and gelatine according to Beijerinck 1 would result in a mechanical 



1 Beijerinck, M. W. Ueber Emulsoidenbildung wiisseriger Losungen gewisser gelatinierender 

 Kolloido. Zeitsch. f. Kolloid-Chem., 7:16. 1910. 



