146 KEITH R. PORTER 



metabolites and the production and deposition of materials important in 

 wall formation. In Fig. 7, e.g., the separation of the cells is a product of a 

 recent division, and primary wall formation is probably in progress. The 

 real purpose of the association is, of course, not indicated. But it is of some 

 value to compare the picture at this cell surface with other instances where 

 wall formation is also in progress and especially where wall formation is 

 more clearly the major activity at the moment of fixation. 



Fig. 8. i\Iicrugraph of section cut in plane of surface t)f onion root tip cell. The 

 section includes a part of the cytoplasm (bottom half) and passes out of cell 

 obliquely through the plasma membrane (p?n) and then into region of wall (czv). 

 The small densities with a ring around them represent cross sections of plasma- 

 desmata (/)). The component of particular interest here is the endoplasmic 

 reticulum and its distribution just within the plasma membrane. It obviously 

 consists of many tubular units interconnected to form a reticular structure which 

 is contiguous with the inner surface of the plasma membrane (see arrow). 



One example of this latter situation is encountered in the formation of 

 the cell plate [47]. As is well known, this structure is characteristic of 

 dividing plant cells and first appears in light microscopy as a row of small 

 pectin vesicles arranged along the equator of the telophase spindle [48]. 

 In the electron microscope image the vesicular phase of plate development 

 is preceded by another structure which has the following developmental 

 history. 



