262 



METAMORPHOSIS 



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of elasticity (' plastic growth '). In proportion as the surface increases the 

 thickness must decrease, and since in nature no such decrease in thickness can 

 as a rule be observed it must be assumed that simultaneously with this super- 

 ficial extension a deposition of new layers, or an increase in thickness from 

 within, takes place. This conception, when carefully studied, denies real sur- 

 face growth altogether, and recognizes only a passive stretching and deposition 

 on the membrane. In sharp contrast to this theory is the second, which 

 explains surface growth by assuming the intercalation of new materials between 

 those already existent ; osmotic pressure on this view acts mechanically by 

 pulling apart the finest particles of the cell-wall and thus favouring the inter- 

 polation of new particles between them. 



These two opposing views have long been known as the theories of * appo- 

 sition ' and 'intussusception ' respectively, and it is only recently that it has 

 come to be believed that both methods in all probability occur in nature. We will 

 illustrate this by reference to a few examples. 



In a cell of Oedogoniiim which is about to divide we observe, near one end 

 and closely applied to the inside, a thickening of the wall in the form of a ring-like 



pad, manifestly composed of two different chemi- 

 cal substances as seen in section (Fig. 51, /). The 

 central mucilaginous region of the pad is the first 

 to be laid down {Schl), and is covered later by a 

 cellulose layer (C) which has the same characters 

 as the remainder of the cell- wall. Apparently, 

 owing to swelling of the mucilaginous core the old 

 cell-wall is torn asunder by a circular crack, 

 and the whole cell is in this way greatly 

 increased in length. The material forming 

 the pad becomes extended so as to form a 

 cylindrical uniting zone between the two older 

 regions of the cell-wall, in such a way that the 

 mucilaginous region forms the external and the 

 J^ia^l;^ (aft'L^HrN"). V;!fXt\Z cellulose part the internal layer of the interca- 

 tinous pad ; c, the cellulose covering of the lated piccc. Contemporaneously with theelonga- 

 ^^ ' tion of this interpolated cylinder (Fig. 51, //, ///) 



there is an obvious decrease in the thickness of the wall, and the whole process 

 gives one the impression that the stretching is purely passive and results from 

 the action of osmotic pressure. On this account Oedogonium has been usually 

 regarded as an example of 'plastic growth'. Closer investigation shows us, how- 

 ever, that the case is not quite so simple as it seems. The new membrane at 

 first is stretched in the longitudinal direction only, and the transverse diameter 

 of the cell remains unaltered or even decreases. Again, it is worthy of note that 

 the growth ceases when the intercalated portion has become quite thin. 

 If the entire growth of this portion depended only on plastic extension, one 

 would be led to expect that the stretching must continue, and all the more easily 

 the thinner the membrane became. If stretching be the factor in the case at all, 

 then changes in the elasticity of the membrane must occur during that process, 

 calculated to render the membrane gradually less extensible. Even of this 

 simple and apparently purely mechanical growth phenomenon we are far 

 from having arrived at a satisfactory explanation. It would seem to us as if 

 the physiological aspect of the case has not as yet received the attention it 

 deserves. 



Berthold has observed intercalary growth in other Algae also, which in 

 many respects suggests that seen in Oedogonium, e. g. in a member of the Con- 

 fervaceae. The cell structure of this alga is illustrated schematically at Fig. 52. 

 Each cell consists of two pieces, x-shaped in longitudinal section, each becom- 



