274 VEGETABLE 



Such are the circumstances, and facts, concerning the pores 

 on the surface of the leaves of different plants. 



But the most wonderful and curious part of their structure 

 still remains to be described, namely, the interior vessels, to 

 which these superficial pores lead. The nature, and form of 

 these, can only be discovered by placing a thin slice of a leaf, 

 cut in the direction of its thickness, under a powerful micro- 

 scope. The leaf of the Clove Pink (Dyanthus caryophyllus,) 

 is well adapted to this purpose. 



A section of such a leaf, highly magnified, is represented 

 by Fig. 246, the pore a being di- F ' 216 - 



vided longitudinally. It is a 

 short funnel-shaped tube, pene- 

 trating the cutis b, surrounded at 

 the bottom by a hollow ring, and 

 opening into the oval vesicles c, 

 d, and e, which as seen by the 

 figure, communicate with each 

 other. The oblong cells e e, are 

 cuticular air vessels, and corres- 

 pond to the meshes, which lead from one pore to another, 

 as shown in the previous figures. On each side of the pore 

 a, are two other pores of the same kind. 



Functions of the Pores of Leaves. Decandolle, Mirbel, 

 Bonnet, and Sprengel, regard these pores merely as absorb- 

 ing, and exhaling vessels. But it is not easy to conceive 

 how the same aperture should perform such opposite func- 

 tions, and a closer investigation has seemed to prove that they 

 do not absorb, though they exhale fluids. 



That they are exhalants, seems to be proved by the fol- 

 lowing facts and circumstances. Aqueous transpiration is 

 greatest in those leaves which have the greatest number of 

 pores. 



By reference to the above table, it will be observed, that 

 these apertures, in the leaves of trees arid shrubs are situated 

 only on the under sides, while in herbaceous plants they are 

 found on both sides. Hence when plates of glass are ap- 

 plied to the under sides of the leaves of trees, they are soon 

 covered with drops of water, but when applied to the upper 

 surfaces they prove that no transpiration takes place there. 

 When the under surfaces of such leaves are varnished, so as 

 to obstruct the pores, aqueous transpiration fails entirely. 

 The Hydrangea (juercifolia, a common hot-house plant, is a 



