f STRUCTURE AND USES OF THE STOMAT^. 121 



pearance of an overlying membrane, without a central chink, is remarked, 

 its adventitious formation may be inferred. 



Whatever may be the difficulty of circumscribing into a definite 

 function, the process of respiration in vegetables, physiologists can no 

 longer doubt the tendency of facts in relation to the use of the sto- 

 mata. It suggested itself to me, that if some expedient were contrived 

 to suspend the functions of the stomata, a sort of negative or patholo- 

 gical evidence might be obtained in regard to their uses. The most 

 effectual means, after some trials, were found in common varnish. If the 

 superior surface of an aquatic leaf, to which it is known the stomata 

 are confined, be well coated with varnish, after the lapse of a few days a 

 distended, unhealthy, and aetiolated state will supervene. Under the con- 

 ditions of this experiment, it is evident that two important consequences 

 follow : That, firstly, the process of exhalation is rendered impracti- 

 cable by the impervious coating, inducing thus an undue retention of 

 water in the Parenchymatous cells ; and that, secondly, since the 

 decomponent agency of the light continues, the liberated oxygen is 

 equally retained, and, if the diseased appearance of the leaf be admitted 

 as criterion, proves deleterious to its organism. Time and opportunity 

 have not allowed me to correct and multiply these observations. 

 So far as my acquaintance with botanical literature extends, I am not 

 aware that this mode of experimenting upon the stomata has been anti- 

 cipated. From various physiological considerations, the accuracy of the 

 analogy cannot be doubted, which makes the stoma to the pneumatic 

 or respiratory system of the plant, what the spiracle is to the tracheal 

 apparatus of the insect. Although the former description of M. De 

 Candolle, that the spiral-vessels communicated by open mouths with 

 the stomatal cavities, has been denied by high authorities in modern 

 phytology, from the experiments of Dutrochet, Dumas* and Dr. 

 Boucherie, it is obvious that the spiral-vessels derive their supply of 

 atmospheric air by means of the stomata. In the leaf of the Holly, which, 

 upon maceration, separates into two layers, I have satisfied myself that the 

 spiral-vessels accompany only the excurrent woody fibres, and are there- 

 fore confined to the upper lamina of the leaf. A parallel to this ar- 

 rangement is found in the structure of some insects, in which the tra- 

 cheal tubes follow only the centrifugal currents of blood. In the 

 Holly, therefore, the communication with the stomata at the inferior 

 surface, can only be indirectly through the intercellular passages. 



Comptes Rendus de TAcademie des Sciences, 1840. 



