PLATE III {continued) 



Fig. 3. The appearance of bordered pits in conifer tracheids under conditions 

 as in Fig. 2. For explanation, see text, p. 64. 



Fig. 4. A piece of Valonia cell wall under the polarizing 

 microscope between crossed Nicols (plane of vibration of 

 polarizer parallel to short edge of page). Note the varia- 

 tions in brightness over the field of view; this implies a 

 variation in m.e.p. See text p. 95. 



