58 



THE CELL. 



form, and arrangement of the prominences or depressions which 

 may occur in cell-membranes. Many of them will become fa- 

 miliar in the course of future study. 



Cell-membranes in Tissue. In tissues the membranes, if pres- 

 ent, appear as partitions separating the cells. In young tissues, 

 the partitions usually appear single (Fig. 1) ; but as the tissue 

 grows older the partitions split along the middle so as to form two 

 layers, each of which may be regarded as the membrane of the 



a 



FIG. 29. (After De Bary.) Elongated cells (ladder-cells, or "scalariform tracheids"), from 

 the underground stem of a fern (Pteris aqnilina), having a series of transverse pits and 

 slits through the walls: , part of a cell seen en face; b, membrane more highly magni- 

 fied; c, section of the wall before the pits have broken through; rf, shows how the pits 

 become slits. 



cell which it surrounds (Fig. 2). The splitting of the partitions 

 is sometimes incomplete or does not take place at all. In the 

 latter case no trace of the double nature of the partitions can be 

 seen. If the separation is incomplete (as in Figs. 2 and 9), angular 

 intervals known as intercellular spaces are often left between 

 the cells. There is every stage between a complete independence 

 of the membranes and their complete fusion. But whether 

 separation be complete or not the partition- wall lying between 



