THE CELL-THEORY 



269 



I 



(C). After the disappearance of the endoplast, changes commence in 

 the periplastic substance or wall of the cell (a), more or less circular 

 or spiral thickenings (c) taking place in it, so as to form the well-known 

 fibre-cell of the sphagnum leaf ; and at the same time, a process of 

 resorption occurs in particular parts of the wall, so that round 

 apertures are formed (d). Nothing can be more instructive than 

 this case, the leaf being composed of a single layer of delicate and 

 transparent cells, so that there are no 

 interfering difficulties of observation ; ^^ 



and we see demonstrated, in the most 

 striking manner, that the endoplast or 

 primordial utricle has nothing to do 

 with the metamorphoses which occur 

 in the periplastic substance. The dis- 

 appearance of the primordial utricle 

 in cells which are undergoing thicken- 

 ing was, in truth, long ago pointed 

 out by Von Mohl ; but neither he 

 nor any of his successors seem to 

 have noticed how completely this fact 

 does away with that activity of the 

 primordial utricle, and passivity of the 

 cell-wall, which they all assume. We 

 have here, in fact, the cell-wall com- 

 mencing and carrying through its 

 morphological changes after the pri- 

 mordial utricle has completely disap- 

 peared, and we see that the so-called 

 secondary deposit in this case is a 

 morphological differentiation of the 

 periplast, which at the same time 

 exhibits its peculiar powers by set- 

 ting up a resorption of its substance at another point. Here 

 however, we have no marked chemical differentiation ; for an 

 instance of which we may turn to the collenchyma of the beet-root 

 (fig. I, A.) There is no question that, at one period of its develop- 

 ment, the whole periplastic substance here, as in the Sphagnum, 

 was homogeneous, and of the same chemical constitution. In the 

 fully formed beet-root, however, we have no less than three com- 

 pounds disposed around each cell cavity. The periplastic substance 

 has, in fact, undergone both a chemical and a morphological differ- 



Portions of the leaf of Sphagnum. A, 

 from the base ; B, more towards the 

 point ; C, fully formed, a, endo- 

 plasts which disappear ; d, those 

 which remain ; <:, spiral thickenings 

 of periplast in the cavities of tlie 

 former ; d, apertures formed by 

 resorption. 



entiation — the innermost layers (c) consisting of ordinary cellulose; 



