38 
ORGANOGRAPHY. 
BOOK I. 
they do exist, they are called Intercellular passages. They 
necessarily follow the course of the tissue, being horizontal, 
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 
Tropaeolum 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 beau- 
tiful 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, replete 
with the 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 ?'e- 
servoirs accidentels of De Candolle ; and also the reservoirs 
