128 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[27. 



CHAPTER II. 



THE TISSUES. 



27. The Connexion of the Cells. As already explained 

 (p. 121), in the development of a tissue, whether by free cell-for- 

 mation or by cell-division, septa are formed, that is, walls which 

 are common to contiguous cells ; these are very thin at first, and 

 appear under the highest magnifying power as a simple plate. 

 As the walls increase in thickness (see p. 103) and acquire a 

 more or less distinctly stratified structure, as seen in transverse 



section, the net-work of primary 

 septa stands out from the thickening 

 layers proper to each cell (Fig. 87). 

 The primary septum between any 

 two cells is now distinguished as the 

 middle lamella (sometimes also termed 

 intercellular substance); it attains a con- 

 siderable bulk at points, where several 

 septa meet at an angle (Fig. 88). Its 

 chemical composition essentially re- 

 sembles that of the tissue in which it 

 is examined. In a tissue where the 

 thickened cell-walls consist of cellu- 

 lose, the middle lamella simply con- 

 sists of a denser, more resistent form 

 of cellulose; in lignified, cuticularised, 

 or mucilaginous tissue, the middle 

 lamella presents the chemical charac- 

 teristics of the tissue in the highest 

 degree, since it is more completely 

 modified chemically than the rest of 

 the cell- wall ; or, in other words, since 

 it contains less residual cellulose than the other layers of the wall, 

 whether they be lignified, cuticularised, or mucilaginous, as the 

 case may be. 



When, as is generally the case, the cells of the tissue have 

 pitted walls, the pits of any one cell are exactly opposite those of 

 the cells contiguous to it (Fig. 87), and are closed by the middle 

 lamella which is, however, penetrated by the protoplasmic fibrils 

 connecting the energids of the respective cells (see Fig. 45, p. 91). 



FIG. 86. Formation of tissue by 

 cell-division in the cortex of the 

 growing stem of Vicia Faba ( x 300). 

 a a The delicate septa have just 

 been formed ; fc nucleus. 



