THE CELL-MEMBRANE. 363 



cubical in shape. As the cells are left behind, and from 

 the growing rather than the dividing zone, they become 

 vacuolated and osmotic pressure gradually becomes con- 

 siderable. When the growth of the cells in turn has ceased, 

 the osmotic pressure causes the membrane to be in a state 

 of tension, so that the limit of its extensibility is reached 

 and its elasticity is equal to the stretching force inside. At 

 this time the calcic pectate at the angles of the polyhedral 

 cells becomes gelatinous and under the strain it gives way at 

 the points where the gelatinisation is complete. The 

 middle lamella in fact ruptures at the angles, while the other 

 layers remain intact. 



This mode of regarding the formation of intercellular 

 passages enables us easily to understand how some of them 

 become coated with a thin layer of pectates. Into some of 

 them at the anofles the gelatinised middle lamella can be 

 seen to extrude, forming little plugs or projections. There 

 may further be a continuation of the passage across the 

 membrane of the soluble pectates, which, not now being 

 balanced by a similar stream from an opposing cell, make 

 their way to the exterior of the membrane and appear as a 

 thin coating over the surface of the passage. Hence may 

 also be formed the curious projections of pectates which 

 have been found as already mentioned in the parenchyma 

 of many Ptendopkytes and other plants. 



BIBLIOGRAPHY. 



(i) WiESNER. Untersuchungen liber die Organisation der vege- 

 tabilischen Zellhaut. Sitz. -Akad. d. math, natur., ci. xciii., 

 bd. ii., Abth., Wien., 1S86. 



(2) RUSSOW. Sitdier der Dorpat. Naturfors.Gesell. Sept., 1883. 



(3) Gardiner. On the Constitution of the Cell-wall and Middle 



Lamella. Proc. Cainb. Phil. Soc, vol. v., pt. ii. 



(4) Braconnot. Recherches sur un nouvel acide universelle- 



ment repandu dans tous les vegetalcs. Aim. de Ch. et de 

 Phys.^ t. xxviii., ser. 2., 1825. 



(5) Paven. Analyse de la partie corticall de TAilanthus glandu- 



losa. Anil, de Ch. et de Phys., t. xxvi., ser. 2, 1824. 



