252 PLANT ANATOMY. 
belliferee, Composite and Conifers ' (which are very rich in such 
examples) of being traced back to the production, extension, and 
prolongation of intercellular spaces. Very frequently four cells. 
Fie. 169. Fie. 170. Fie. 171. 
Figs. 169-171.—Four boundary cells, gggg, in Fig. 176 receding from each other; in 
Fig. 171 the intercellular passage so produced exerts a pressure upon the boundary 
cells, 
Fig. 169, gggq) recede from each other in the region where they 
meet together. 
The boundary cells (y), which in the beginning (Fig. 170) 
still project with their conyex walls into the resin-passage, 
Fia. 172. Fia. 173. 
Fias. 172-173.—Beginning of the depression of the boundary cells, which in Fig. 173 
are strongly compressed in a radial direction (Miller). 
recede (Fig. 171), and are more and more depressed in a radial 
direction (Figs. 172, 173). At the same time, in the boundary 
cells, and frequently also in the farther surroundings of the 
' Compare also Fig. 106. 
