72 RECEPTACLES OF SECRETION. 
b. Air Cavities.—In water-plants the intercellular spaces are 
frequently of large size, and bounded by a number of small cells 
regularly arranged (jig. 174), by which they are prevented from 
communicating with one another, or with the external air; they 
are then commonly termed air 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 and Umbelli- 
ferous plants, which have been formed by the destruction of their 
internal tissues by the more rapid growth of the outer portions ; 
these large cavities are termed lacunx, and appear to have no 
special functions to perform. 
c. Receptacles of Secretion.—In many plants, again, the in- 
tercellular canals or spaces act as receptacles for the peculiar 
secretions of the plant ; in which case they are termed Recep- 
tacles of Secretion. In many instances these are closely allied 
to the internal glands (figs. 170 and 171) already described, 
and are frequently confounded with them ; indeed, some regard 
them as highly developed forms of internal glands. These 
receptacles vary much in form, but are usually more or less 
BireAt7o- 
fig. 175. Fruit of Parsnip (Pastinaca sativa). A. Dorsalsurface. a.b.b.c. ¢ 
Primary ridges. 1, 2, 3, 4, Vittae. B. Horizontal section of the fruit. 
The letters and figures refer to the same parts as in A. In Jig A. the 
vitte are readily seen by noticing that they are shorter than, and alter- 
nate with, the ridges, a, 6, b, ¢, c. 
elongated. They are formed by certain cells separating from 
each other as they are developed (schizogenous), by which means 
canals and spaces of various kinds are formed in the surrounding 
tissue. In the Coniferee they contain turpentine, and have 
therefore been termed turpentine vessels. In the plants of this 
order they occur especially in the wood (fig. 186, la) and bark : 
those in the wood forming elongated tubular passages. In the 
pericarp of the fruit of Umbelliferous Plants they form the 
receptacles of oil, which are commonly termed vitte ( Jig. 175, 
1,2, 3,4, Aand B). The receptacles of secretion are found es- 
pecially in certain orders of plants, to which from the nature of 
