THE GROWTH OF THE CELL 263 



ing thinner towards the middle of the cell and having these thinner regions 

 overlapping. The protoplasm next deposits a median band of membrane, thick 

 in the centre and thinning off towards either end. Union of the two consti- 

 tuents renders the wall of the cell of equal thickness throughout. When the cell 

 starts growing the overlapping borders of the external membrane separate 

 and the inner membrane comes up more and more to the surface of the cell- 

 filament. After a transverse wall has been formed this inner wall takes on the 

 x-form and new inner membranes appear in each daughter-cell. Similar 

 observations on other Algae have been made by many other investigators 

 (compare Berthold, 1886, and Knut Bohlin, 1897). The difference between 

 this case and Oedogoniuni lies chiefly in the fact that the deposition layer does 

 not grow so suddenly in the present case, and hence the idea of purely me- 

 chanical stretching is excluded. 



Phenomena similar in principle have been demonstrated as taking place 

 in apical as well as in intercalary growth. Thus Schmitz 

 (1880) and Strasburger (1882) have observed at the 

 growing apex of Bornetia secundiflora, one of the Flori- 

 deae, a special kind of lamination which is figured at Fig. 

 53. The growing point is made up of successive lamellae ^-^ ^, structure of the 

 of limited thickness which increase in superficial extent ceii-waii of Microspora 

 and burst through similar older lamellae, which thus osqTpi. I'^Fig.'^srx^^c^"'' 

 become bevelled off at a certain distance from the 



apex. Noll (1887) successfully stained the cell-walls of Derbesia, Caulerpa 

 and other marine Algae with prussian-blue and thus differentiated these layers 

 from those succeeding them. These experiments make it certain that deposition 

 of new lamellae takes place at the growing point, by whose superficial growth 

 the older lamellae are burst. We may certainly assume from 

 a study of such observations that the separate lamellae then 

 exhibit no further growth where they have been torn through, 

 or do not, at least, grow as vigorously as the newly- 

 formed lamellae. The older lamellae are, doubtless, passively 

 stretched, but whether the young lamellae grow pas- 

 sively also cannot be decided from the experiment. If in 

 this and other similar cases the young cell-wall also suffers 

 passive stretching we should naturally attribute it to osmotic Fig. 53. Lamination 

 activity ; but it is of importance to note that this osmotic Borneiia^ecu>id(^or^, 

 pressure is insufficient of itself to stretch the membrane plasti- x 7^- After Stras- 



n • -4- • 1 ^U J. U X Ul BURGER (J882, PI. IV, 



cally, smce it is known that much greater pressures are unable Fig. 55). 

 to extend the cell-wall beyond its limits of elasticity (Pfeffer, 

 1892, 241). Again, we never find membranes in the living cells which are 

 stretched beyond the limits of their elasticity. We may assume, however, 

 with Noll (1895) that plastic stretching is possible without overstepping the 

 bounds of elasticity. It should be remembered that a bent wooden bow 

 gradually unbends, a fact which can be due only to an internal concentric 

 layering. In this unbent condition, however, the bow at any moment can be 

 again elastically bent. Similarly, in the cell- wall stretched by turgor pressure, 

 an unbending and even a plastic extension might arise without the wall be- 

 coming thereby inelastic. It is often assumed that the protoplasm has an 

 effect on the elasticity of the membrane, but we have no grounds for such an 

 assumption. 



It was for long believed that surface growth in the cell-wall arose only by 

 deposition of lamellae and plastic stretching of these by turgor pressure, but more 

 recently many arguments have been advanced against this view ; moreover, 

 there are individual cases which have been thoroughly examined, and to which 

 this explanation cannot be rightly applied. There is first of all the apical growth 



