62 



ORGANOGRAPHY. 



Fig. 151. 



Fig. 151. Air-cells or 

 cavities from the stem of 

 Limnocharis Plumieri. 



are then commonly termed air-cells or 

 cavities. In such plants these cavities fulfil 

 the important services of enabling them 

 to float, and of supplying their interior 

 with air. In other instances we find large 

 air cavities, as in the stems of Grasses, 

 Eushes, and Umbelliferous Plants; these, 

 which have been formed by the destruction 

 of the internal tissue by the more rapid 

 growth of their outer portions, have no 

 direct functions to perform. 



Receptacles of Secretion. — In many plants 

 again, we find that the intercellular 

 canals or spaces act as receptacles for the 

 peculiar secretions of the plant ; in which case they are termed 

 Eecejptacles of Secretion. In many instances these are closely allied 

 to the internal glands already described {figs. 14:6 and 147), 

 and are frequently confounded with them ; but properly speak- 

 ing, an internal gland is a secreting organ in itself, while a 

 receptacle of secretion is merely a cavity containing a secretion 

 which has been formed in other parts and deposited in it. 

 These receptacles vary much in form, but are usually more 

 or less elongated. In the Coniferse they contain turpentine, and 

 have therefore been termed turpentine vessels. In the plants of 

 this order they occur especially in the wood and bark ; those in 

 the wood forming elongated tubular passages. In the rind of the 

 fruit of Umbelliferous Plants they form club-shaped receptacles 

 of oil, which are commonly termed vittce {fig. 152). These re- 



A Fig. 152. 



Fig. T.52. Frmt ot Fursniv) (Pastinica sativa). A. Dorsal surface. B. Hori- 

 zontal section of the fruit, a, b, b, c, c. Primary ridges. 1, 2, 3, 4, 5, 6. 



Vittae. 



ceptacles are found especially in certain orders of plants, to which 

 from the nature of their contents they communicate important 

 properties. 



Intercellular Substance. — The spaces above described as occur- 

 ring between the sides of cells, appear in some few cases to be 



