Botanical Collections, 181 



organs, the function of which is a matter of conjecture, and the actual 

 structure of which has given rise to much difference of opinion. These 

 organs have received the names of pores, glands, or stomata, according to 

 the views of different observers j and while one class of botanists has consi- 

 dered them of unknown function and structure, others have contended, that 

 they are of the nature of pores, and that their office was, according to the 

 one, to facilitate evaporation; to the others, to assist in the process of 

 respiration. Their function is obviously of so obscure a nature, that no 

 direct experiments are likely to demonstrate exactly what it is ; but their 

 structure is a point upon which observation may be expected to cast some 

 light. Mr Bauer long ago represented these organs in the wheat, as perfo- 

 rations opening into a minute subcutaneous cavity, and as destined to afford 

 a direct passage into the interior of a plant for those minute fungi, whose 

 ravages are so well known in the form of what the farmers call the mildew 

 in corn. Other observers have, however, doubted whether the supposed 

 perforations always existed ; and Mr Lindley, in his lectures in the Univer- 

 sity of London, has repeatedly expressed his difficulties upon the subject. 

 The fact is, that they are so minute, the tissue of which they consist is so 

 exceedingly transparent, and it is so difficult to examine them, except by the 

 aid of transmitted light, that it is not, perhaps, possible to determine posi- 

 tively, in all cases, whether a perforation exists or not. 



A memoir, by M. Adolphe Brongniart, upon the structure of leaves, and 

 on their relation with the respiration of vegetables in air and water, read before 

 the Academy of Sciences of Paris, throws some light upon the subject. The 

 author states, that the leaves of plants that live in the air have a totally different 

 structure from those that are completely submerged, and that this difference 

 in the structure of organs is in direct relation to the two principal functions 

 of leaves, respiration and transpiration. In leaves exposed to air, the 

 surface of the leaf is covered by an epidermis of uncertain thickness, formed 

 of one or more layers of colourless cellules, closely packed together. 

 This membrane is pierced with the above-mentioned pores, or stomata. 

 The doubts that have been entertained upon the existence of perforations 

 in these stomata, M. Brongniart thinks he has removed, and that it is 

 certain, that in the centre of each stoma is an opening, by which the outer 

 air communicates with the parenchyma. This parenchyma is evidently the 

 seat of respiration ; for it is the part which changes colour in exercising this 

 function, which becomes green by the absorption of the carbon of the 

 carbonic acid of the atmosphere, and which is discoloured again in darkness 

 by the combination of the carbon of its juices with the oxygen of the air. 

 This parenchyma differs entirely from that of other organs, by the numerous 

 irregular cavities that it contains, which communicate with each other and 

 the outer air by means of the openings of the stomata. It is into these 

 cavities, in the cavernous parenchyma of aerial leaves, that the atmospheric 

 air penetrates when it is absorbed by the surface of the utricles of the 

 parenchyma, that are distended with the fluids which seem to nourish the 

 plant. According to M. Brongniart, aquatic leaves, if submerged, differ, 

 in being completely destitute of epidermis. It is not stomata alone that 

 they want, as has long been known, but the epidermis also. There are none 

 of the cavities that abound in the parenchyma of aerial leaves j but, on the 

 contrary, the cellules of the tissue are compactly fastened together without 

 any interstice, and the air dissolved in the water can only act on their outer 

 surface. For this reason the proportion borne by this surface to the whole 

 mass of the leaf is unusually great ; the leaves, from want of epidermis, dry 

 up quickly when exposed to the air, and can only exist in water, or a very 

 humid atmosphere. Hence the author concludes, that the epidermis is 

 destined to protect aerial leaves against too rapid evaporation, and the stomata 

 or pores of this epidermis become necessary to mainttiin a communication 

 between the atmosphere and the parenchyma. 



