THE LIVING CELL 33 



powers of the microscope, and will pass through a filter paper, though 

 they cannot pass through the minute pores of parchment, as already 



mentioned. 



In the dry state colloids are often characterised by a high affini 

 for water, leading to imbibition when placed in water. In some cases 

 the colloid dilutes itself so far as to go into solution, as does gelatin. 

 In others the colloid, though imbibing water to a considerable extent, 

 remains in a semi-solid, jelly state. This is the case with the cellulose 

 of the wall. The swelling of dry seeds in water is due to imbibition 

 both by cell-walls and by protoplasmic colloids. 



Many colloidal solutions, while liquid at higher temperatures or 

 in greater dilution, set to form a jelly on cooling or on concentration. 

 Gelatin and starch paste are examples. The fact that protoplasm 

 exists sometimes as a liquid, sometimes as a jelly, points to its 

 colloidal nature. Protoplasm appears to consist of an extremely complex 

 system, in which proteins are present in colloidal solution in an aqueous 

 medium containing also mineral salts and other soluble substances. 

 Associated with the proteins are fats and other constituents, such as the 

 protoplasmic catalysts or enzymes. They will be considered in Chapter 

 VIIJ. The surface of colloidal particles, presented to the aqueous 

 medium, is the seat of important phenomena. Crystalloidal substances 

 which may be present in the water tend to accumulate on the surface 

 of the particles by a process known as adsorption. This is likely to 

 accelerate any chemical reactions that may occur between different 

 substances adsorbed side by side on colloidal particles ; and there 

 is little doubt that the great chemical activity of protoplasm is in 

 part due to this adsorption, especially on to the surface of the proto- 

 plasmic catalysts or enzymes. 



It is a matter for surprise that many different chemical reactions 

 should be able to proceed at once within the minute confines of a 

 cell. As will be seen later, some of these reactions are known to be 

 restricted to the plastids, and it is often assumed that there is an 

 invisible separation of the protoplasm into further separate areas, 

 each devoted to a particular type of reaction. The nucleus is the 

 bearer of the hereditary characters of a plant, as is described later. 

 It must also play an important part in the other activities of the cell, 

 directing them in such a way that the plant as a whole, in a suitable 

 environment, acquires those morphological and physiological attri- 

 butes which characterise its race. 



B.B. 



