ANATOMY OF TISSUES. (ai 
olands of this system limited to certain cells and groups of cells. 
On some stems these groups consist of a large number of cells, 
as in Silene, Populus, and others. Usually the gland is com- 
posed of a small cluster of cells, as on leaves of Clereodendron, 
where they le just under the epidermis, which is broken by 
their action. Their position and the breaking of the epidermis 
combine to give them the appearance of a leaf parasite. The 
greater number of epidermal glands are hairs or outgrowths of 
a single cell. These seldom remain simple, but become multi- 
cellular. The glands of insectivorous plants are not included 
here, but require special treatment. 
INTERNAL GLANDS. 
These consist of so-called internal gland-hairs which push 
their way through into the intercellular spaces, and there ex- 
erete their contents. Such are found in leaves of Lathraceae, 
also in the midrib of the leaf of Aspidium Felix-mas. Another 
class of internal glands is composed of the parenchymatic cells 
which surround the ends of the vascular bundles where they 
terminate in the leaf. 
Of the second class, or those holding their contents, the 
tubes are certain cells so filled with their secretions that nothing 
of the protoplasm remains or can be detected. They are in some 
cases elongated tubular cells, in others nearly isodiametric. The 
nature of the secretions varies in different plants, being gen- 
erally either gums, resins, mucilage, or crystals. Secretions of 
the latter class are apt to be found in cells called tubular ducts 
from the fact that they are situated in rows and the transverse 
walls are absorbed, making a true duct like those of the vascular 
bundle. Raphides are often found in these ducts. The inter- 
cellular spaces, which are utilized as organs of secretion, arise 
in two ways. Either the cells are separated by splits in the 
walls, along the line of the middle lamellae, beginning at their 
