42 THIRD REPORT 1833. 



is divided completely by a number of partitions caused by the 

 ribs and jirincipal veins, so that the air cavities have not actually 

 a free communication in every direction through the parenchy- 

 ma, but are to a certain extent cut off from each other. This 

 is conformable to what M. Mirbel has described in Marchantia, 

 who finds the leafy expansions of that plant separated by par- 

 titions into chambers, between which he is of opinion that 

 there is no other communication than what results from the per- 

 meability of the tissue *. 



The statement of M. Adolphe Brongniart, that all leaves in- 

 tended to exist in the air are furnished with a distinct cuticle 

 on their two faces, while those which are developed under 

 w^ater have no cuticle at ail, has not been disproved, unless in 

 the case of Marchantia f , whose under surface can scarcely be 

 said to have a distinct cuticle; but this jilant, which can only 

 exist in humid shady places, is perhaps rather a proof of the 

 accuracy of the theory of M. Brongniart than an exception 

 to it. 



That the stomata in all cases open into internal cavities in 

 the leaf, where the tissue is extremely lax and cavernous, ap- 

 pears also exti'emely probable. It was especially found to be 

 the case by M. Mirbel in his so often quoted remarks upon 

 Marchaniia. 



With regard to the stomata themselves, no one appears yet 

 to have confirmed the observation of Dr. Brown;}:, that their 

 apparent orifice is closed up by a membrane. On the contrary, 

 the observations of M. Mirbel on Marchantia, if they are to 

 be taken as illustrative of the usual structure of those singular 

 organs, go to establish the accuracy of the common opinion 

 that the stomata are apertures in the cuticle. That most skil- 

 ful physiologist, while watching the development of Marchantia, 

 remarked the very birth of the stomata, which he describes as 

 taking place thus : — The appearance of a little pit in the middle 

 of four or five cells placed in a ring is a certain indication of 

 the beginning of a stoma. The pit evidently increases by the 

 enlargement and separation of the svirrounding cells. If the 

 nascent stoma consists of five cells, of which one is surrounded 

 by four others, then the central one is destroyed; but if it con- 

 sists of three or four cells adjusted so as to form a disk, then 

 the stoma is caused by the separation of their sides in the cen- 

 tre, by which means a sort of star is created. It is true that 



* " Reclierclies Anatomiques et Pliysiologiques sur le Marchantia poli/ruor- 

 pha," in Nouvcaux Annalcs du Museum, vol. i. p. 7. 

 + Ibid. p. 93. 

 J Suppl. prlmttm Pradromi Floros Novce IloUaiidicc, p. 3. 



