FUNCTIONS OF THE INTERCELLULAR SYSTEM. 789 
dermis. Even when the division is complete, a portion (if the 
leaf is examined ina superficiak position) still remains as asimple 
lamella. These two sister- or daughter-cells are called guard- 
cells, and further differ from the rest of the epidermis in 
containing chlorophyll and starch. 
A special form of stomata called water-pores is found at the 
termination of the veins of some leaves (Tropxolum &c.) which 
differ from other stomata in being non-contractile, and in some 
cases the guard-cells become entirely absorbed, leaving open 
spaces. 
5. FUNCTIONS OF THE APPENDAGES OF THE EPIDERMIS. — 
Hairs and their modifications appear to be designed to protect 
the epidermis and parts beneath from injury due to cold and 
other external influences, hence we find young buds (see page 
105), &c., frequently coated with hairs, also in many flowers to 
prevent injurious insects carrying away pollen. Hairs also ap- 
pear in certain instances, at least to some extent, to absorb fluid 
matters from the atmosphere, whilst in other cases they serve to 
assist the epidermis in restraining exhalation ; and we find, ac- 
cordingly, that plants which are densely coated with them are 
well adapted to grow in dry situations, and to sustain without 
injury a season of drought. 
Glands are those organs which in themselves secrete some 
peculiar matter. (See page 68.) These secretions are either 
permanently stored up in them, or excreted. 
6. FUNCTIONS OF THE INTERCELLULAR SystTEM.—The inter- 
cellular canals, except at those times in which the tissues of the 
plant are gorged with sap, as in the spring of the year, are filled 
with air, and the especial function which they perform is to allow 
a communication between the external air and the contents of 
the internal tissues by virtue of the laws regulating the diffusion 
of gases. They likewise facilitate exhalation of liquid matters 
by their connexion with the stomata. The itercellular spaces 
are also, in most cases, filled with air, though certain recent 
observers have described protoplasm as occupying some of them, 
and as communicating with the protoplasm of the cells ; while 
the air-cells and air-cavities, as their names imply, are in like 
manner filled with aeriform matters, and in water-plants are 
especially designed to diminish the specific gravity of the parts 
in which they are found, and thus to enable them to float 
readily, or to be suspended in the water. The receptacles of 
secretion, as their name implies, contain the peculiar secretions 
of certain plants, and are closely allied in their nature to glands. 
(See page 72.) 
