NECTARIES.—INTERCELLULAR SYSTEM. 71 
very common in many other plants besides those already men- 
tioned: thus in all the Labiate Plants, as Mint, Marjoram, 
Thyme, Rosemary, Sage, &c. ; and it is to the presence of 
the secretions they contain that such plants owe their value 
as articles of domestic economy, or as perfumes, or medicinal 
agents. 
Holding a sort of intermediate position between the internal 
and external glands as above described, are the true nectaries 
of flowers, which being strictly of a glandular nature will be 
most properly alluded to here under the name of nectariferous 
glands. ‘They are well seen at the base of the petals of the 
species of Ranunculus (fig. 172) and in the Crown Imperial 
(fig. 173). These glands consist of a pore or depression into 
which a honey-like fluid or nectar is secreted, or rather ex- 
creted, by the surrounding cells. The tissue of the stigma of 
Flowering Plants is also covered by a viscid secretion or 
excretion at certain periods, and may be considered therefore 
as of a glandular nature. The surface of the ovary and other 
parts are also sometimes more or less covered by a similar sac- 
charine fluid, and are then described as nectariferous. 
When glands or other receptacles containing peculiar secre- 
tions arise from the separation of uninjured cells from one 
another, they are termed schizogencus ; when from the absorption 
of a mass of tissue, lysigenous (fig. 171, 7, 7). 
6. INTERCELLULAR System. — Having now described the 
different kinds of cells, and the modifications which they 
undergo when combined so as to form the tissues, we have in the 
next place to allude to certain cavities which are placed between 
their walls, or produced by the destruction of some of their 
component cells. These constitute the Intercellular System. 
a. Intercellular Passages or Canals and Intercellular Spaces.— 
The cells being, in the great majority of cases, bounded by 
rounded surfaces, or by more or less irregular outlines, it must 
necessarily happen that when they come in contact with one 
another they can only touch at certain points, and therefore 
interspaces will be left between them, the sizes of which will 
vary, according to the greater or less roundness or irregularity 
of their surfaces. When such spaces exist as small angular 
canals running round the edges of the cells and freely commu- 
nicating with one another, as is especially evident in round 
or elliptical parenchyma (jig. 62), they are called intercellular 
passages or canals ; but when they are of large size, as in stellate 
or spongiform tissue, they are termed intercellular spaces (figs. 
93 and 124, c). In most cases these spaces and canals are filled 
with air, and when they occur in any organ exposed to the 
atmosphere in which stomata are found, they always communi- 
cate with them (fig. 132, 1), by which means a free passage is 
kept up between the atmosphere and the air they themselves 
contain. 
