THE VEGETATIVE ORGANS OF PLANTS. 289 
The particles of atmospheric air have easy access to the 
interior of all leaves, however dense and close their epi- 
dermis may be, however few or small their stomata. All 
leaves are actively engaged in absorbing and exhaling cer- 
tain gaseous ingredients of the atmosphere during the 
whole of their healthy existence. 
The entire plant is, in fact, pervious to air through the 
stomata of the leaves. These com- 
municate with the intercellular 
spaces of the leaf, which are, in 
general, occupied exclusively with 
air, and these again connect with 
the ducts which ramify throughout 
the veins of the leaf and branch 
from the vascular bundles of the 
stem. In the bark or epidermis of 
woody stems, as Hales long ago 
discovered, pores or cracks exist, 
through which the air has communi- 
cation with the longitudinal ducts. 
These facts admit of demonstration by 
simple means. Sachs employs for this pur- 
pose an apparatus consisting of a short wide 
tube of glass, B, fig. 59, to which is adapted, 
below, by a tightly fitting cork, a bent glass 
tube. The stem of a leaf is passed through 
a cork which is then secured air-tight in the 
other opening of the wide tube, the leaf itself 
being included in the latter, and the joints 
are made air-tight by smearing with tallow. 
The whole is then placed in a glass jar con- 
taining enough water to cover the projecting leaf-stem, and mercury is 
quickly poured into the open end of the bent tube, soas nearly to fill the 
latter. The pressure of the column of this dense liquid immediately forces 
air into the stomata of the leaf, and a corresponding quantity is forced on 
through the interecllular spaces and through the vein ducts into the 
ducts of the leaf-stem, whence it issues in fine bubbles at S. It is even 
easy in many eases to demonstrate the permeability of the leaf to air by 
immersing it in water, and, taking the leaf-stem between the lips, produce 
a current by blowing. In this case the air escapes from the stomata. 
‘The air passages of the stem may be shown by a similar arrangement, 
13 
