THE CELL- WALL. 481 



in the case of spores, pollen, &c., they often become quite spherical. 

 But if they form part of a permanent tissue, the expansion of the 

 organ of which they form part stops at a certain point, before they 

 cease to swell, and thus the mutual pressure comes to bear upon 

 them and causes the production of plane surfaces. 



We may trace this by making sections of a pith of a shoot of Elder 

 from the growing point, or pxnctum vegrtationis, downwards : at the paint 

 the nascent cells are squarish ; lower down they have swollen into sphe- 

 rical, while when full-grown they are dodecahedral. The similar change 

 from cylindrical to prismatic takes place in the cambium-cells of annual 

 stem and shoots ; but in succeeding years the cambium-cells formed by 

 division of preexisting cells exhibit a rectangular outline first and last, 

 only increasing in diameter, chiefly in a radial direction. 



Dimensions. The magnitude of cells is very varied. About T J- F 

 of an inch may be taken as an average of the diameter of paren- 

 chyma-cells ; the cylindrical cells are especially remarkable for the 

 great length they often acquire as contrasted with their transverse 

 diameters, and with the transverse and perpendicular diameters of 

 other forms. 



The larger cells of the pith of the Elder are about -^-^ of an inch in 

 diameter, but ^ is to be regarded as a large diameter in parenchyma. 

 On the other hand, the spores of Fungi affjrd examples of extremely 

 minute dimensions, such as ? Vo to -g-^g TT f an i ncn - The cylindrical 

 cells of wood are not uncommonly ^ of an inch in length ; liber-cells 

 sometimes from -fa to or of an inch (Flax). Hairs composed of one 

 or more cylindrical cells, and the cylindrical cells of some of the Confervre, 

 especially VaucTieria y Srycpsis, &c., and Chara, also attain longitudinal 

 dimensions to be measured in inches, while their diameter is estimated in 

 hundredths of an inch. 



The Cell-wall. In all young cells the wall is of membranous 

 nature, and in many cases it always retains this character. \V T hile 

 young this membrane is freely permeable by water, elastic and 

 flexible. As the cell-\vall grows older it becomes altered in con- 

 sistence and firmer, opposing a greater obstacle to the entrance of 

 water into its substance, independently of any great increase of 

 thickness, as we see in cork-cells : when it increases in thickness 

 it may remain soft and flexible, or become very dense ; but in such 

 cases it generally remains tolerably freely permeable by water, even 

 when most dense, while the softer kinds absorb water so readily 

 that they swell up considerably when wetted. 



Membrane of living cells always appears to contain water as an essential 

 part, almost like the water of crystallization in hydrated salts. When 

 dried, cells contract more or less ; and many phenomena of bursting of 

 fruits, sporanges, &c. are the result of the tearing down of weak regions 

 of cellular tissue by the contraction of firmer tissues in drying. Cellular 



