PLANT CELL GROWTH AND NUTRITION 469 



chromosomes and of cytoplasm but also indicates that DNA may aflFect 

 less directly, the extranuclear synthesis of protein, because it may aflFect 

 RNA in both the nucleus and the cytoplasm. It is very clear that protein 

 synthesis can occur in enucleated cells; it is equally clear that no enu- 

 cleated structure has any hope of permanence, and therefore even the 

 behavior of the enucleated cells has its basis in the prior and nucleated 

 organization. 



It seems evident, therefore, that the role of the plant growth sub- 

 stances that exert their eflFects upon cell division (with its concomitant 

 protein synthesis) and/or upon cell enlargement must operate in this 

 area where nuclear-cytoplasmic eflFects are operating. 



Submicroscopic structure of cytoplasm. It is now necessary to re- 

 examine some of the ideas mentioned above in the light of current work 

 on cells. The trend now is to indicate clearly that cells are extremely 

 heterogeneous structurally, and that metabolically they are highly com- 

 partmentalized, in such a way that even similar metabolites may do 

 diflFerent things in diflFerent parts of the same cell (Steward, Bidwell, 

 and Yemm, 1956 ) . 



The extreme heterogeneity of the cytoplasm of plant cells is now 

 evident from the examination of critically fixed cytoplasm in cells 

 which admittedly are poor in vacuoles, for their presence so compli- 

 cates the techniques of electron microscopy that good pictures have not 

 yet been obtained with this type of cell. The cytoplasm of animal cells 

 lends itself well to these studies, and excellent pictures are available 

 in Frontiers of Cytology, edited by Palay ( 1958 ) . At this point, how- 

 ever, one may pardonably assume that the sparse cytoplasm of the 

 vacuolate plant cells should have somewhat similar structure to that 

 which has been examined in the more meristematic cells by Whaley 

 and others ( 1959 ) . These instructive new techniques shed the follow- 

 ing light upon the ion-accumulation problem. 



It seems no longer feasible to think only in terms of well-ordered, 

 multi-layered, rather static surface membranes which allow solutes to 

 pass through pores into the more or less liquid and homogeneous cy- 

 toplasm and across which the solutes move, by some activated but 

 free diflFusion, into the vacuoles at the inner surface. The current pic- 

 ture (see Figure 6) is that the cytoplasm is traversed by a system of 

 canals— the endoplasmatic reticulum— which seems to connect with 

 both the outer ( especially at plasmodesmata ) and the inner surfaces— 

 especially the nucleus. If these canals are real channels of transport, 

 we may have to look for strictly localized sites of entry into the cell and 

 localized channels of transport through the ground substance of the 

 protoplasm, as well as a much more discrete mechanism of entry into 

 the vacuole, than has previously been conceived. Both mitochondria 

 and Golgi bodies represent vesicular inclusions in the cytoplasm from 



