74 VEGETABLE ORGANOGRAPHY. 



round spots. These rosettes are visible on the inferior 

 surface of the leaves of Begonia spatulata, formmg 

 little dots, which are visible to the eye ; they are espe- 

 cially very remarkable in the leaves of Crassula cordata 

 and C. arhorescens ; for the round dots which are seen 

 by the naked eye, are the collections of stomata. This 

 fact made me formerly suggest the idea that the stomata 

 micrht be the orifices of the vessels ; for each of the dots 

 of the Crassulas, which I have just mentioned, is the 

 termination of a fibre which is itself a bundle of vessels. 

 I was confirmed in this idea, from considering that the 

 stomata are not present in plants devoid of vessels ; I 

 confessed, however, that I never saw the continuation 

 of a vessel to one. Comparetti has gone farther than 

 I have, and has asserted that he has seen vessels termi- 

 nate in stomata. Mirbel says, on the contrary, that the 

 stomata are the orifices of cellules ; and Kieser considers 

 them as the terminations of the intercellular canals. I 

 incline, now, to this last opinion ; but it is a question 

 which deserves fresh examination. 



The stomata are absent in several plants, on account, 

 it seems, of their manner of living. Thus — 1st, They 

 are not found either on the leaves or stems of plants 

 which grow under water, such as Zostera, Ceratophyl- 

 lum^ &c. ; and in those which have part of their organs 

 under and part above water, as several species of Pota- 

 mogeton, Myriophyllum, Nymphcea, &c. the stomata 

 exist only in the parts exposed to the air ; they are 

 found on the leaves of Ranunculus aquatilis when they 

 are raised above the water, but are wanting when they 

 grow under it. 2dly, The part of the leaves of bulbous 

 plants which is concealed in the Onion, and conse- 

 quently etiolated, is either entirely deprived of them, or 

 presents some closed and imperfect ones. AU truly 

 parasitical vascular plants, which are not of a green 



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