VEGETABLE PHYSIOLOGY: ELEMENTARY ORGANS. 
13 
parent, with a dark centre, whereas the supposed pores of 
granular tissue have a solid appearance, and are perfectly 
opaque. Woody tissue constitutes the greater proportion of the 
ligneous part of all plants; it is common to the bark, and forms 
a considerable portion of the veins of leaves, to which it gives 
firmness and tenacity. 
Besides these elementary parts, there exists also in plants 
certain organs, which are composed of similar forms of tissue : 
these are termed intercellular passages , receptacles of secretion, 
air-cells , and raphides. 
Intercellular passages are the spaces formed by the 
imperfect adhesion of the vesicles of cellular tissue. They vary 
in form according to the pressure of the cells ; thus, if this be 
very regular, the spaces will be very small or altogether oblite¬ 
rated, whilst, on the other hand, if the pressure is strongest in 
any particular direction, the spaces will become horizontal, 
vertical, or oblique, according to the direction of the angles of 
the cells. Their size also varies considerably, for in plants of a 
dry or rigid character they are frequently very small, whilst in 
those of a more succulent nature they are so large as to present 
the appearance of cells ; in aquatic plants they are generally 
very large in the horizontal partitions, which separate the air- 
cells, and have been supposed to act as passages for the ready 
access of air from one to another. 
Receptacles of Secretion are the intercellular passages in 
a state of distension by the accumulation of proper sap, which, 
when unusually increased, ruptures the coats of the adjoining 
tissue, and by this means the cavities are formed. In some 
plants these are tubular; in Conferee they are very irregular in 
figure, and even in position, chiefly forming hollow cylindrical 
spaces in the bark. Those contained in the rind of the orange 
and lemon are oblong and spherical; from their situation they 
are easily examined, and give an accurate idea of the organ. 
Air-cells are another kind of intercellular passages, similar 
to the last, but, instead of being filled with the juices of the 
plant, these contain only air. This is provided for by their con¬ 
struction : in the former (that is the receptacles), the orifices of 
the cells are all open, and thus the juices drain readily from one 
to the other, whilst in the latter they are closed up, except in 
the partitions which divide them from each other, and thus the 
discharge of fluid into them is prevented. They cannot be re- 
