Natural History, 8fc. 421 



is caused by the ravages of a kind of fungus. ' The axis which 

 supports the glumes and floral organs of grasses, is formed of elon- 

 gated cellular tissue, the cellules of which are placed close together, 

 without sensible intercellular passages, and of fibro- vascular bundles 

 of false trachea? or ducts, and spiral vessels ; in the fleshy mass, of 

 which the smut consists, no structure of this sort is visible, at what- 

 ever time it is examined ; but, for examining it satisfactorily, I have 

 taken the plant at the earliest period when the spike is capable of being 

 examined. At this time the fleshy mass is found to consist entirely 

 of an uniform tissue, containing uniform four-sided cavities, separated 

 by partitions formed of one or two layers of very minute cellules. 

 These cavities, which, in organization, resemble the regular Iacuna3 

 observable in the cells of aquatic plants, are filled by a compact 

 homogeneous mass, composed of very small granules, perfectly sphe- 

 rical and uniform in size ; they were slightly adherent to each other, 

 and of a greenish colour in spikes but little developed distinct, or 

 simply clustered towards the centre of each mass, and of a pale 

 nut colour, in spikes which were a little developed : finally, at a 

 more advanced period, the cellular partitions disappear, the glo- 

 bules separate completely, and the whole mass is transformed into a 

 cluster of powder, formed of very regular globules perfectly alike, 

 black, and quite analogous to the reproductive bodies of other 

 fungi.' 



8. STRUCTURE OF LEAVES. 



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

 and on their relation with the respiration of vegetables in air 

 and water, has been read before the Academy of Sciences of Paris. 

 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, respira- 

 tion 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 toge- 

 ther. This membrane is pierced with the pores usually known by 

 the name of 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 that 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 





