CHAP. I. 
AIR CELLS. 
39 
en ccecum of the latter, which are the clavate vessels of oil 
found in the coat of the fruit of Umbelliferae, and which are 
commonly called vittce. Although the receptacles of secretion 
have no proper coat, yet they are so surrounded by cellular 
tissue, that a lining or wall is formed, of perfect regularity and 
symmetry. The tissue of this lining is generally much 
smaller than that of the neighbouring parts. When filled 
with a fluid having a different refractive power from that of 
the surrounding parts, they give a semitransparent dotted 
appearance to the organs in which they occur, as may be seen 
by holding up the leaf of an orange tree against the light. 
While, however, many kinds of receptacles of secretion are 
mere cavities in the tissue, others are little nuclei of cells, as 
in the Dictamnus (Jig- 10. c). These are of the nature of 
glands, and are called internal glands by Meyen. 
Numerous modifications of these parts have been described 
by the German anatomists, especially by the last-mentioned 
author, but they only relate to the refinements of the subject. 
In figure, the receptacles are extremely variable, most com- 
monly round, as in the leaves of the Orange and of all Myr- 
taceae, where they are called crypta^ or glandulce impresscE^ or 
reservoirs vesiculaires, or glandes vesiculaires^ or receptacles of 
oil. In the Pistacia Terebinthus the receptacles are tubular ; 
in Coniferae they are very irregular in figure, and even position, 
chiefly forming large hollow cylindrical spaces in the bark. 
Those in the rind of the orange and lemon are little oblong 
or spherical cysts ; their construction, which is easily examined, 
gives an accurate idea of that of all the rest. 
3. Of Air Cells. 
Besides the common intercellular passages, and the recep- 
tacles now described, there is another and a very remarkable 
sort of cavity among the tissue of plants. This is the air 
cell ; the lacuna of Link. Like the receptacles of secretion, 
the air cells have no proper membrane of their own, but 
are built up of tissue; and this sometimes takes place 
with a wonderful degree of uniformity and beauty. Each 
D 4 
