PART II. ANATOMY AND HISTOLOGY. 



[26 



CHAPTER II. 



THE TISSUES. 



26. The Connexion of the Cells. According to their 

 arrangement in space, the following combinations of cells may be 

 distinguished : 



A. Filaments, where the cells are connected only by their con- 

 tiguous ends, and so form a filament, e.g., many Algae, as Spirogyra 

 (Fig. 65), (Edogonium (Fig. 62), and many hairs. 



B. Surfaces, when the cells form a single layer and are in con- 

 tact in two directions of space (length and breadth), e.g. many Algae 

 and the leaves of many Mosses. 



C. Masses, when the cells are in contact on all sides. 



The tissues commonly consist of cells which have originated from 

 common mother-cells by their repeated division into two, and which 

 have been connected from the first in consequence of the mode of 

 formation of the septa (Fig. 60). In a few special cases tissues are 

 formed otherwise (spurious tissues) ; either cells which have been 

 hitherto isolated become adherent and then continue their growth 

 in common ; or filaments consisting of rows of cells become inter- 

 woven and exhibit a common growth, without however having be- 

 come adherent in every case (Fig. 64 sK). 



The Common Wall of cells combined into a tissue is, in the 

 first instance, usually extremely thin and delicate, and appears 

 under the strongest magnifying 

 power as a simple plate (Fig. 60). 

 As it increases in thickness a 

 middle lamella usually becomes 

 visible (Fig. 66), which divides 

 the wall into two parts, one of 

 which apparently belongs to each 

 of the contiguous cells. This 

 middle lamella is nothing more 

 than a specially differentiated part 

 of the wall which belongs to both 

 of the cells in common. Its chemi- 

 cal composition, which is different 

 to that of the remainder of the 

 wall, permits of its solution (in 

 nitric acid and chlorate of potash), 



FIG. 66.- Transverse section of the 

 cortical cells of Trichomaneg speciosum 

 (x600). Middle lamella (m) ; ti the 

 cell-wall adjoining the lamella; I cell 

 cavity; bordered-pits which meet in 

 adjoining cells ; the pits on each side 

 are divided by the middle lamella. 



