THE ENDOPLASMIC RETICULUM 1 43 



relative to the structure and function of this newly defined system — again 

 based on animal material. Obviously if these are to be accepted as prin- 

 ciples of cytology, they should be equally applicable to plant cells. There- 

 fore a good deal of interest has been displayed in electron microscopy of 

 plant cells. The early attempts were less than satisfactory because of the 

 technical problems encountered. These early observations were, however, 

 of sufficient value to show that the plant cell had in common with the 

 animal a membrane limited system, homologous to the ER of animal cells 

 [44, 24]. Since that time, technical developments have provided improved 

 images which leave no doubt that plant cells of several kinds have in their 

 fine structure organelles and systems identical in many respects to those 

 found in the animal units [44a, 45,]. Thus the nuclear envelope shows 

 the characteristic two membranes and pores, and continuity at certain 

 points with the cytoplasmic part of the system. Cisternal units of the ER, 

 though randomly oriented in the meristematic cells of the root (Fig. 7), do 

 achieve patterns of organization in cells in more advanced stages of 

 difi'erentiation (Fig. 6). The RNP particles in plant cells do not show the 

 extreme affinities for cisternal surfaces that one notes in animal cells, but 

 particles are attached to some parts of the system and not to others so that 

 one can recognize rough and smooth divisions (Fig. 6). 



The functions of the ER in the plant cell are of course not more clearly 

 shown by a few pictures than they were in the animal equivalent. However 

 the material lends itself very readily to controlled experimental study and 

 one can reasonably hope that a substantial amount of new information may 

 come from this source. Then also certain features of interest are peculiar 

 to plant cells, and one can hope to exploit these for clues to function not 

 displayed to the same advantage in animal cells. At the outset we can 

 reasonably assume that the little we have learned from animal material can 

 be transferred to the plant. 



One structural association especially has held our interest and it is to 

 this that attention will be specificallv directed (see Porter and Machado, 

 [46]). 



The majority of the structures representing the ER in meristematic 

 plant cells are large, lamellar units. When these reach out to the cell 

 surface they show at their margins distinctive changes in morphology. 

 These are reflected in the thin section by changes in the dimensions of the 

 profiles. Thus, instead of being long and narrow, they appear short and 

 often circular, which indicates that at this level the svstem is constructed 

 of tubular rather than lamellar units (Fig. 7). These are joined together at 

 this level into a reticular structure which can be more readily visualized 

 when included in the plane of the section (Fig. 8). Again, for the morph- 

 ologist this suggests a functional involvement of the ER in events taking 

 place at the cell surface. These events include of course some exchange of 



