82 • FUNDAMENTALS OF CYTOLOGY 



tobacco mosaic virus passes readily through walls having plasmodesms, 

 but not into the guard cells of stomates, where plasmodesms are appar- 

 ently absent. The transfer of materials from cell to cell by way of the 

 very slender plasmodesms has not been directly observed, although 

 mass movement of protoplasm has been seen occurring through larger 

 pores having a diameter of 1.5 to 2/i in the green alga C odium. In the 

 red algae, peculiar cell connections have been described, but actual 

 protoplasmic continuity here is still subject to doubt. 



It is probable that the functional significance of plasmodesms is to 

 be found in the conduction of stimuli promoting correlation and in the 

 transfer of certain materials important in metabolism. The same 

 interpretation is warranted for the intercellular bridges in animal tissues. 

 By virtue of such protoplasmic continuity a tissue or a complex organism 

 would seem to be better able to function consistently as an individual 

 than it w^ould if only nonprotoplasmic materials separated its protoplasts. 

 At the same time it is to be remembered that certain correlating factors, 

 e.g., electrical gradients and the diffusion of dissolved substances, can 

 exist in systems that are partly or even wholly nonprotoplasmic. 



The Formation of Cellulose by the Protoplast.^ — This topic is at 

 present a highly controversial one, but it is included here because of its 

 prominence in cytology today and its great biological interest. For 

 many years the prevailing view has been that cellulose first becomes 

 visible at the surface of the protoplast, where it is deposited as successive 

 layers having the crystalline structure described in foregoing pages. 



According to an opposing view, which has come into prominence 

 during the past decade, cellulose first appears in the cytoplasm in the 

 form of minute ellipsoidal bodies having a size of about 0.5 by 1.5^. 

 These form chain-like aggregates and are built into the wall along with a 

 colloidal material that cements them together. Moreover, the wall can 

 be broken down into such ellipsoidal bodies by the use of hydrochloric 

 acid and centrifuged out of the mixture. 



In recent papers on the ellipsoidal particles it is claimed that they are 

 produced by plastids. In the green alga Halicysfis ring-like masses of 

 carbohydrate are formed just beneath the membranes of disc-shaped 

 chloroplasts. These break up into "mercerized" cellulose particles 

 of uniform size which are liberated into the cytoplasm when the plastid 

 membranes disintegrate. They are then built into the developing 

 wall in successive layers. In the cotton fiber there are small disc- 

 shaped colorless plastids in which cellulose particles appear in a similar 

 manner. In the green cells of the leaves and stem of the cotton plant, 

 both starch-forming chloroplasts and cellulose-forming leucoplasts 

 function simultaneously. 



