490 



PHYSIOLOGY. 



In many wood-cells, especially in root-structures, the reticulated or 

 scalariform cells have the meshes so small that they become in fact pitted 

 cells. 



Tertiary Layers. In some cells both kinds of thickening occur, 

 so that it is convenient to distinguish tertiary growths. In the 

 wood-cells of the Tew (fig. 542), of the Lime, and other plants the 

 secondary layers are pitted, and a tertiary growth subsequently 

 appears in the form of a spiral fibre. 



The pits on the walls of contiguous cells correspond, and they do not 

 generally occur opposite intercellular spaces, or on the outside of epidermal 

 cells; but exceptions occur to both these rules, to the latter especially 

 in the leaves of Cycas. The first rule has much influence on the marking 

 of the large cells forming part of ducts, which are often in contact with 

 several cells, one above another, and with parenchyma-cells, other ducts, 

 or with intercellular spaces, on different sides. In the wood-cells of 

 Conifers, the peculiar bordered pits occur only on the sides parallel to 

 the medullary rays, not on the internal and external walls. 



Cellulose, &c. Cell-membranes, including the secondary layers, 

 are composed of the substance called cellulose, which is one of a class 

 of organic compounds intimately connected as regards chemical 

 constitution, but presenting remarkable physical differences. Of 

 these compounds the most important are : sugar and dextrine, 

 soluble in cold water, and occurring in solution in the cell-sap ; 

 starch, insoluble in cold, but softening and swelling into a mucilage 

 in boiling water, and found in the form of granules in the cell- 

 contents ; and cellulose, insoluble in cold or boiling water, alcohol 

 or ether, obstinately resisting the . _, 



action of alkaline solutions, but soluble . ^ 



in strong sulphuric acid, and forming 

 the permanent solid parts of vegetable 

 structure. This cellulose is supposed 

 to be derived from one or other of the 

 materials above mentioned. ' 



Cell-membranes, originally composed of 

 pure cellulose, undergo changes at sub- 

 sequent periods which alter, in a marked 

 manner, their behaviour towards chemical 

 reagents ; and it is not at present certainly 

 ascertained what is the real cause of the 

 series of modifications which they present. 

 If we compare the membrane of a nascent 

 cell, of thick-walled parenchyma, the solid 

 and often dark-coloured walls of the cells 

 of old heart-wood, of liber-cells, the very 

 resistant membranes of corky tissues, and the layers of gelatinous or 

 cartilaginous consistence so abundantly developed in the larger Algae, we 

 meet with extremely different characteristics, as to the explanation of 



Wall of the cells of the liber of 

 Cocos : a, primary membrane ; 6, 

 oldest secondary layers; c, more 

 recent secondary " layers ; the 

 layers marked b are strongly in- 

 crusted. Magn. 600 diam. 



