482 PHYSIOLOGY. 







tissues with soft thick membrane, like those of the Algae &c., contract in 

 drying so as to cause the shrivelling of the structure. All such tissues 

 absorb water when wetted, and swell up again, but do not in all cases 

 reassume their original flexibility. Cells of wood, liber, &c. also expand 

 when wetted ; but the expansion takes place in a direction transverse to 

 their axes, and they usually contract in the longitudinal dimension as 

 they swell laterally. Hence, although wood and fibrous structures swell 

 in water, it is only in the direction across the grain, and cordage simul- 

 taneously contracts in the direction of the fibres. 



Diluted sulphuric acid and alkaline solutions cause a swelling of the 

 membrane of most cells, of which advantage is sometimes taken in w r oven 

 fabrics to render the stuff' closer in texture. By soaking in an alkaline 

 solution, the single fibres are made to swell so as to come more completely 

 into contact and fill up the interstices. 



Primary, unaltered cell-membrane is colourless ; subsequently it 

 becomes coloured, usually of a tint of brown, apparently by infil- 

 tration of substances formed in the contents, since by boiling the 

 membrane of old, deep-brown tissues with nitric acid, or with 

 solution of potash, the colouring-matter may be extracted. 



The original membrane of a newly formed cell is, as far as we 

 have the means of perceiving it, a homogeneous layer of substance, 

 the porous nature of which is, in most cases, only to be concluded 

 from the fact of its permeability, no visible pores, except in excep- 

 tional cases, being revealed by the most perfect microscopes we 

 possess. 



It is important to note this homogeneity of the primary cell-wall, as 

 .the membrane almost always becomes marked with dots and spiral lines, 

 indicating inequality of thickness, as it becomes thicker. 



This primary membrane is apparently a secretion from the pro- 

 toplasm, though by some it is looked on simply as a chemical pre- 

 cipitate. It appears to have the property of growing by what is 

 called intussusception of molecules, since it expands to accom- 

 modate the increasing contents of the cell in cell-growth, with- 

 out any indication of structure necessarily accompanying the 

 expansion. 



No better example of this can be mentioned than the growth of the 

 pollen-tube of Phanerogamia, which sometimes acquires a length of 2 

 or more inches (Cactus] without ever departing from the homogeneous 

 pellicular structure. Cell-membrane, however, may increase in size by ex- 

 pansion, as we see in the cell-division of GEdor/oniitm, in which a thickened 

 ring of accumulated cellulose is stretched out by the elongating cell and 

 becomes a thin membranous coat to the latter. 



Molecular structure. The molecular structure of cell-membrane has 

 been studied by Nageli, who, from his researches on the constitution of 

 the membrane of the starch-grain by means of polarized light, comes to 



