pp 



i^ 



f- 



m 



cw 



'^12 



Fu;. 12. This is identified as the external margin of an epidermal cell of the 

 onion root tip. The nucleus is at A^, a nuclear envelope at ne. Organelles of the 

 cytoplasm include mitochondria (ni), dictyosomes (d), proplastids (pp), and 

 endoplasmic reticulum (er). The surface of the protoplast and its relation to the 

 extracellular wall (nu) are the points of major interest. It appears that the wall has 

 been put down in layers, reflecting, possibly, phases of wall formation. The surface 

 of the protoplast in this instance shows no continuous plasma membrane but only 

 short fragments. In places (arrow), the ends of ER elements project beyond the 

 surface defined by these membrane fragments as though blending with the extra- 

 cellular material. We interpret this image as descriptive of a process of wall 

 formation in which a cortical layer of the cell, with fragments of the contained 

 organelles, is externalized by the development of a new plasma membrane behind it. 



