CHAPTER II 



The Form of the Plant Cell 



BEFORE PROCEEDING to a discussion of the modern developments in 

 the molecular architecture of the walls of plant cells it will be as 

 well to pass briefly in review the range of structures, in a microscopic 

 sense, with which we shall be concerned. This is all the more desirable, 

 even for those who have already some considerable knowledge of plant 

 anatomy, since the form in which the material is presented in nature is 

 the only form available for study; so that, unlike the state of affairs in 

 corresponding investigations with matter not associated with living 

 things, it is impossible to change to any extent the form of the experi- 

 mental material. This imposes strict limits upon the kind of things 

 which can be done, particularly in view of the inherent microscopical 

 complexity of even the smallest piece of a tissue which can conveniently 

 be handled. 



All plants, like all animals, originate from individual drops of proto- 

 plasm which are commonly spherical, or nearly spherical, in shape and 

 are usually such as can comfortably be seen only under a microscope. 

 Such a shape undoubtedly conforms to the liquid, or semi-liquid, 

 consistency of the protoplasm, at least during some stage of its develop- 

 ment. In plants, however, unlike the animals, this drop of protoplasm 

 comes, sooner or later, to be covered by a membrane, thin and delicate 

 yet undoubtedly solid, which therefore limits any further rapid changes 

 in shape; and so long therefore as the cell, as we may now call it, re- 

 mains free from its neighbours and removed from any other obstacle 

 to development, it will remain spherical, or approximately so, provided 

 that the membrane — the primary wall — is such that it will expand 

 equally in all directions in response to the same mechanical disturbances. 

 In fact, many cells which do remain free in this way, do retain their 

 almost spherical shape — some unicellular algae, spores of fungi, mosses, 

 ferns, etc. — although in some cases — the filamentous algae for instance 

 — they rapidly become cylindrical in spite of the absence of neighbouring 

 lateral cells. 



Such instances as these latter naturally imply that the surrounding 

 solid envelope is not isotropic and this we shall have to examine in some 



II 



