EEPKODUCTIVE ORGANS OF PLANTS. 313 



The entire phmt is, often, pervious to air through 

 the stomata of the leaves. These communicate 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 communication with the longitudinal ducts. 



Tliese facts admit of demonstration by simple means. Saclis employs 

 for this purpose 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 smear- 

 ing with tallow. The whole is then placed In a glass jar containing 

 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 lat. 

 ter. 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 ciurent by blowing. In this ease the air escapes 

 from the stomata. 



The air-passages of the stem may be shown by a similar arrange- 

 ment, or in many instances, as, for example, with a stalk of maize, by 

 simply immersing one end in water and blowing into the other. 



On the contrary, roots are destitute of any visible 

 external pores, and are not pervious to air or vapor in 

 the sense that leaves and young stems are. 



The air passages in the plant correspond roughly to 

 the mouth, throat, and breathing cavities of the animal. 

 We have, as yet, merely noticed the direct communica- 

 tion of these passages with the external air by means of 

 microscopically visible openings. But the cells which 

 are not visibly porous readily allow the access and egress 

 of water and of gases by osmose. To the mode in which 

 this is effected we shall recur on subsequent pages. 



The Offices of Foliage are to put the plant in com- 

 munication with the atmosphere and with the sun. On 



