68 BOTANY 



96. The Cell-wall in Tissues. In tissues the walls which 

 separate contiguous cells are at first simple and homogeneous. 

 The plate of cellulose which first forms between two sister 

 masses of protoplasm in cell-fission is a single one, the com- 

 mon property, as it is the common secretion, of the proto- 

 plasm masses. As the wall becomes older and thicker, and 

 stratification takes place, it shows a line of separation into 

 two halves ; this may become so well marked as actually to 

 result in the splitting of the wall, as is the case in succulent 

 tissues when, on account of a particular kind of tension, 

 intercellular spaces are formed in the angles between the 

 cells (Fig. 51). 



97. By a still further differentia- 

 tion, after a considerable thickness of 

 the wall has been attained, there 

 may arise a common middle lamella, 

 which appears at first sight to lie 

 between the original cell-walls (Fig. 

 52). This middle lamella, which is 

 simply the result of a particular 

 stratification, was long mistaken for 

 an intercellular substance, and two 



Fig. 52. Cells of the woody ,i 111 - f\ 



part of the young stem of theories were held as to its nature. On 

 the one hand, it was supposed to be 

 an original common matrix, in which 

 After Sachs. ^he cells themselves were imbedded ; 



and on the other, it was held to be of the nature of an ex- 

 cretion from the surrounding cells into the intercellular 

 spaces. The first of these theories was possible only so long 

 as the knowledge of the origin and development of cells was 

 exceedingly defective. The second theory is rendered ex- 

 tremely improbable by our present knowledge of the mode 

 of growth of the cell-wall by intussusception. 



Until recently another view has been largely held, name- 

 ly, that the middle lamella was to be regarded as the original 

 common wall of the cells, and that the remaining portions 

 were after-deposits upon it. This view gave rise to the terms 

 Primary Cell-wall and Secondary Cell-wall, which are still 

 used to some extent. As this explanation of the structure 

 rests upon the all-but-abandoned theory of the thickening 



