STRUCTURE.] TURPENTINE VESSELS. 93 



zontal, vertical, or oblique, according to the direction of the 

 angles of the tissue by which they are formed. Their size varies 

 according to the size of the tissue and the quantity of sap. 

 In plants of a dry nature, they are frequently so small as to 

 be scarcely discoverable ; while in succulent plants they are 

 so large as to approach the size of cells, as in the stem of 

 Tropseolum majus. They are remarkably large in the hori- 

 zontal partitions which separate the air cells of water plants. 

 In Limnocharis Plumieri they exist in the form of little holes 

 at every angle of the hexagons of which the partitions in 

 that plant consist; and are, no doubt, there intended as a 

 beautiful contrivance to enable air to pass freely from one 

 cavity to another. 



2. Of Receptacles of Secretion. 



Fig. 10. 



But it frequently occurs that the simple intercellular pas- 

 sages are dilated by the secretions they receive, and either 

 increase unusually in size, or rupture the coats of the neigh- 

 bouring tissue ; by which means cavities are formed, filled 

 with sap altered to the state which is peculiar to the particular 

 species of tree producing it. Cavities of this nature are often 

 called vasa propria. To this class also are to be referred the 

 turpentine vessels of Grew ; * the reservoirs accidentels of De 

 Candolle; and also the reservoirs en caecum of the latter, 



* The reader may learn from Grew's words on this subject, how accurate his 

 ideas were upon such points of Vegetable Anatomy as the microscopes of his day 

 enabled him to study. In his Anatomy of Plants, p. 110, Book III., he speaks 



