TISSUES. 45 



expansions are met with in some algae, also in the leaves of 

 most mosses and hepatics. 



Cell-masses. — When the cells are in contact on all sides, 

 thus forming solid masses of tissue frequently of large size, 

 and including every form of tissue not falling under either 

 of the preceding sections. Such masses of cells in the 

 higher plants eventually become profoundly modified in form 

 and function, thereby rendering possible division of labour. 



The Common-wall of cells combined to form a tissue is 

 at first thin and apparently homogeneous ; but as it increases 

 in thickness the central portion or middle lamella becomes 

 differentiated as a thin plate which divides the common-wall 

 into two layers, one belonging to each of the two contiguous 

 cells. The chemical composition of the middle lamella is 

 different to that of the remainder of the cell-wall, and as it 

 is soluble in nitric acid and potassic chlorate — whereas the 

 remainder of the cell-wall is not — its solution admits of the 

 separation of the component cells of firm tissues. 



Intercellular Spaces are cavities between the cells of a 

 tissue, and originate in two different ways. The most 

 usual method is by the splitting of the common-wall of 

 contiguous cells along the line of the middle lamella 

 {schizogenous) and generally commences at the angle where 

 several cells meet ; sometimes whole masses of tissue are 

 separated from each other in the above manner, forming 

 very large spaces called air-chambers, as in the tissues of 

 most aquatic plants. The second or lysigenous type originates 

 from the absorption of masses of tissue, the resulting cavities 

 often containing the product resulting from the disintegration 

 of the cell-walls, as cherry-gum, also various resinous, oily or 

 fragrant bodies, such as those contained in the cavities of the 

 rind of the orange. Such structures were formerly called 



