REVIEWS. 411 



or a " liinitiug membrane " at the end of the growing 

 period. 



The layers of the cell membrane tend to become extended 

 on the inner face of the wall, those of starch on the outer. 

 Hence, in the membrane, each layer is in a state oi positive 

 tension towards the next outer, and negative towards the 

 inner one. The destruction of membranes also teaches the 

 same lesson as in the case of starch, only we have to deal in 

 many cases with more complex la\ers, some of which are 

 altered in various ways. For further facts and discussion 

 we must refer the reader to the book itself, and pass on to 

 say a few words concerning Strasburger's treatment of some 

 further portions of the subject. 



'^Die Protemkrystalle " and " Scheidewand-hildung " form 

 the headings to the next two sections. 



Protein crystals or ''crystalloids" differ from ordinary 

 crystals in being capable of swelling, apparently according 

 to laws similar to those vv^hich govern the changes produced 

 by warming many ordinary crystals. Their form is the same 

 from the commencement ; and since many have been pro- 

 duced artificially, they can no longer be regarded as growing 

 by " intussusception." Strasburger regards them as differ- 

 ing in no important respect in their origin from ordinary 

 crystals. 



In the formation of division walls or laminae Strasburger 

 considers that the elements of the cell-plate (" microsomes ") 

 are not composed of carbohydrates, but approach proteids 

 in their composition, and we are to conclude that they, to- 

 gether with the plasma sheet in which they are embedded, 

 form the septum, much as the thickening layers are formed 

 on a cell'wall. These " microsomes " are carried in the 

 '' Verbindungs faden " (and not on their surface), and form 

 the dots so often described in the figures of the process of 

 cell division. We are therefore to view the streams of pro- 

 toplasm as carrying these " microsomes "" to the equatorial 

 region of the dividing mass, where they become mobilised 

 and arranged into a sheet, which forms later the thin pri- 

 mary division-wall. 



In some cases, as Spirogyra, CladophorUy the mobilisation 

 of the " microsomes " is efi'ected without Verbindungs 

 Jdden y fine streams of " microsomes " move towards the 

 edges of the equatorial ring, and become gradually 

 distributed inwards until a sheet is formed, the " micro- 

 somes" and material between them becoming coherent 

 laterally. 



We are therefore to accept the consequences that the 



