4 INTRODUCTION. 



like a watch-glass upon a watch. It is at this time so delicate 

 that it dissolves in distilled water in a few minutes. It gradu- 

 ally expands, becomes more consistent, and at length so large, 

 that the cytoblast appears only as a small body inclosed in one 

 of the side walls. The portion of the cell-wall which covers the 

 cytoblast on the inner side, is, however, extremely delicate and 

 gelatinous, and only in rare instances to be observed; it soon 

 undergoes absorption together with the cytoblast, which like- 

 wise becomes absorbed in the fully-developed cell. The cyto- 

 blasts are formed free within a cell, in a mass of mucus-granules, 

 and the young cells lie also free in the parent cell, and assume, 

 as they become flattened against each other, the polyhedral 

 form. Subsequently the parent cell becomes absorbed. (See a 

 delineation of young cells within parent cells, plate I, fig. 2, 

 b, b, b.) It cannot at present be stated with certainty that the 

 formation of new cells always takes place from a cystoblast, and 

 always within the existing cells, for the Cryptogamia have not 

 as yet been examined in this respect, nor has Schleiden yet ex- 

 pressed his views in reference to the Cambium. Moreover, 

 according to Mirbel, a formation of new cells on the outside of 

 the previous ones takes place in the intercellular canals and on 

 the surface of the plant in the Phanerogamia. (See Mirbel on 

 " Marchantia," in Annales du Musee, 1, 55 ; and the counter- 

 observations of Schleiden, Miiller's Archiv, 1838, p. 161.) A 

 mode of formation of new cells, different from the above de- 

 scribed, is exhibited in the multiplication of cells by division of 

 the existing ones ; in this case partition-walls grow across the 

 old cell, if, as Schleiden supposes, this be not an illusion, inas- 

 much as the young cells might escape observation in conse- 

 quence of their transparency, and at a later stage, their line 

 of contact would be regarded as the partition wall of the parent 

 cell. 



The expansion of the cell when formed, is, either regular on ; 

 all sides, in which case it remains globular, or it becomes poly- 

 hedral from flattening against the neighbouring cells, or it is irre- 

 gular from the cell growing more vigorously in one or in several 

 directions. What was formerly called the fibrous tissue, which 

 contains remarkably elongated cells, is formed in this manner. 

 These fibres also become branched, when different points of 

 the cell-wall expand in different directions. This expansion of 



