40 THE TISSUES OP PLANTS 



of plane surfaces, as for instance, when tliey assume a 

 cylindrical or retain tjieir spherical form, it is evident that 

 the walls of contiguous cells will only come into contact at 

 certain points of their sm-face, and that triangular spaces 

 will be left between the cells. These intercellular passages 

 are beautifully apparent between the cylindrical cells which 

 constitute the pith of the stem of Anemone Pennsylvanica, 

 and afford the most satisfactory proof that these cells do not 

 form a continuous and homogeneous mass, but are in reality 

 separate cavities aggregated together and communicating 

 with each other through their contiguous walls. 



Fig. 8. 



riangular intercellular canals between tlie cylindrical cells of Anemone 

 PennsylTanica. 



In some instances, however, cells which are hexagonal in 

 outline form intercellular passages, and this in a manner 

 so interesting that it demands a particular description. If 

 a section of the young petiole of Sparganium ramosum be 

 placed beneath the microscope, a number of triangular 

 apertures, known to botanists as lacuna, will be seen ; these 

 are evidently the result of certain notches in the cell walls, 

 which correspond with those in the walls of contiguous 

 cells. As growth progresses these notches become deeper. 

 The lacunaa, a, a, a, (Fig. 9,) enlarge at the expense 

 of the area enclosed by the cells, until at length the cells 



