STRUCTURE AND USES OF THE STOMATA. 119 



this, at least, is the probable inference and if admitted, a proof is 

 afforded of the open condition of the stomata. The injected leaf was 

 then carbonized between plates of glass, and again examined with refe- 

 rence to the condition of the stomata. Surprise was excited when, 

 notwithstanding the proof of the patency, as presented by the escape 

 of the infused air, a pellicle over the stomata, darkened by the charring 

 as in the uninjected leaf, was observed under the microscope. This 

 similarity of result, obtained by processes of experiment essentially dif- 

 ferent, establishes the correctness of my former suspicion, that the 

 membrane is generated in the preparation of the leaf. Nor is this cir- 

 cumstance difficult of explanation, since in all plants a gummy sub- 

 stance, called by the Germans Phycomater or organic mucus, covers the 

 surface and even the cells of the stomata and parenchyma ; constituting, 

 according to the more recent observations of Schwann, the materials out 

 of which cells are originally formed. In reference to the stomatal 

 pellicle, therefore, in the lamina of a fully developed leaf, the explana- 

 tion is most probable which supposes that the air which, always in 

 minute quantities, occupies the intercellular spaces, dilating under the 

 heat of charring, acquires force enough to carry before it a bubble 

 of this mucous substance, the summit of which becomes carbonised 

 when brought into contact with the heated glass. By adhering to the 

 margins of the aperture, it thus presents the appearance of continuity 

 with the surrounding epidermis. In the examination of stomata 

 generally, however, it should be remembered, that in some species 

 of plants this mucous covering assumes the form of a universal cuticle, 

 and by its induration over the stomata forms a distinct unorganised 

 membrane. 



After having multiplied to a few additional instances the observations, 

 of which the results are just stated, it became evident that the stoma- 

 tal organs, even in a limited portion of cuticle, exhibited different cha- 

 racters. By carefully detaching the epidermis from the stem of the 

 Tradescantia, the stomata could be distinguished in a perforate condi- 

 tion, without any variation either in the figure or diameter of the orifices. 

 In making, however, a comparative examination of portions of cuticle 

 stripped from the inferior surface of a maturely formed leaf at the base, 

 and from a small expanding bud at the summit of the stalk, differences in 

 the form of the stomatal organs were again recognised. It was not 

 until my examinations had been extended to several other plants, that I 

 associated these varieties with differences in the age of the leaf from 

 which the cuticle was taken for inspection. On the young and old 

 leaves of the Anchusa angustifolia, and still more strikingly (if the 



