COMBINATIONS OF CELLS. TISSUES. 507 



at least relatively, simple, its constituents being the descendants of 

 a small number of cells. 



The simplest mode of combination of cells is that which is met with in 

 a large number of the Algae of low organization, where the cells are asso- 

 ciated for a time in what are called colonies, tlie members of which are 

 more or less completely independent of each other in physiological respects, 

 but morphologically represent parts of a determinate whole j while ulti- 

 mately they separate, each to lay the foundation of a new colony. 



Examples of this may be seen in the grouped Desmidiea, like Pedias- 

 trum (tig. 503, B, ), the Diatomea, &c., and in the Palmellece ; to this head 

 is also referable the structure of some of the filamentous Confervoids, 

 Volvocineae (tig. 503, D), and Hydrudictyon. These groups of cells are 

 either held together by simple attachment at certain points of their sur- 

 faces, as in the Desnudieoe, Ilydrodictyon, Uiati,ma (tig. 503, B, c), &c., 

 or by their being enclosed in a gelatinous common envelope (resulting 

 from the expansion or the decay of parent-cell membranes), as in the 

 Volvocineo!) Palmellece, and Nostochinece. 



Intercellular boundaries. A complete coalescence of the cellu- 

 lar membrane of one cell with that of its neighbour so as to form a 

 homogeneous whole takes place, so that, although the bounding 

 membrane between one cell and another would appear necessarily 

 to be double, each cell having its own proper cell-wall, yet if very 

 young growing tissues be examined where the cell-walls are very 

 thin, the boundary- wall between adjacent cells may be seen to be 

 simple, without any trace of separation. It is only in older thick- 

 walled cells that a line of demarcation becomes obvious, in the 

 form of an intermediate lamella, at one time spoken of as the inter- 

 cellular substance, and supposed to be a distinct substance, but 

 which is now shown to be the result merely of a difference in den- 

 sity or molecular structure of the cell- walls during their thickening. 



Where the cells, during growth, separate at various points one 

 from another to form intercellular spaces, there the boundary-walls 

 necessarily split to form the spaces in question. !So also where 

 cells originally united become disconnected, as in the pulp of fruits, 

 the partition- walls naturally become separated, though originally 

 the boundary- wall is uniform and homogeneous. 



Parenchyma. The tissues are distinguished into kinds accord- 

 ing to the form of the cells, the character of the cell-membrane, 

 and the manner in which the cells are connected together. 



Where the cells are roundish or elliptical, the tissue is called paren- 

 chyma ; and this is called imperfect or perfect accordingly as the consti- 

 tuent cells have interspaces between them or are closely packed so as to 

 leave no intercellular spaces. Where the cells are much elongated, the 

 tissue is called prosenchyma, and the constituent cells are known as fibres. 

 Cartilaginous tissue is known as collenchyma ; and two other kinds are 



