FORM OF THE PLANT-CELL. 



53 



42 



24. B. INTEHCELLULAB SYSTEM. The contact of the sides 

 of the cells in plants is seldom or never perfect, so that there are 

 formed numerous cavities, of which the following are the most 

 important varieties : 



a. Original cavities formed by the imperfect contact of the cells : 



1. Intercellular passages; small three-cornered canals running 

 around the cells, and seen in almost all kinds of parenchyma. 



2. Intercellular spaces ; great irregular spaces between the cells, 

 occurring especially in spongiform cellular tissue. 



b. Later-formed cavities: 



1. Receptacles of special secretions. These arise from the ex- 

 udation of the juices of the cells into the intercellular passages. 

 Two kinds of these may be distinguished : 



a. Formed by compact cells, 

 lying close on one another, and 

 apparently not separable; as the 

 resin-cells of the bark of Coni- 

 fer &, and individual gum-cells. 



/3. Formed by loose cells, with 

 their walls projecting vesicularly 

 into the cavity, and apparently 

 separable. Such cavities mostly 

 contain peculiar secretions, and 

 are seen in the latex-canals 

 of species of Mammillaria and 

 Rhus, the gum-cells of Cycadea 

 (fig. 42.), and the resin-cells of 

 the wood of Coniferce. 



40 Parenchyma from the stem of Flcia Faba, In the cells are seen porous openings, 

 and in two cells cytoblasts, with granules of starch. 



41 Parenchyma from the bark of Quercus Suber (cork oak), a, longitudinal section. 

 b, as seen from surface. 



4a Tissue from the opening of two dextrin passages in the petiole of Cycas revolnta. 

 The compact parenchyma is clothed with a delicate vesicular tissue, projecting into the 



cavitv. 



