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, 2?, 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, so as 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 intercellular 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 cases 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, 



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