198 PHYTOTOMY. 



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' 'The ap|)eatance of these shts, in certain families, has, how- 

 ever, some other remarkable circumstances, which at least 

 somewhat limit the relation of the spiral vessels to these organs. 

 Among plants of an entirely cellular structure, slits have been 

 actually observed, although they are but rare, in the Marchan- 

 tia, and in some species of Splachnum. Those plants of the 

 higher orders, which have no green leaves, are destitute also 

 of slits. Although, commonly, they appear only on the un- 

 der surface, they are yet observed on both surfaces in the 

 Coronariae, the Grasses, the Palms, and even in Pines. But 

 - they are found only on the upper surface of water plants, 

 whose leaves are spread out flat upon the water, and in such 

 land plants as have their leaves lying flat upon the ground. 

 ; But these slits are found on every leafy integument, pro- 

 vided it be not too much set with hairs. They are according- 

 ly observable on the exterior surface of the calyx, and serve, 

 when the calyx takes the place of the corolla, or is united 

 with it, as an excellent mark of distinctipn between these two 

 coverings of the sexual parts. They are as invariably want- 

 ing in the proper corollar integument, as in the sexual parts 

 themselves, (175.) Yet in one instance they have been ob- 

 served in the epidermis of the Cherry ; (Vom Bau und der 

 Natur der Gewachse, Tab. IX. Fig. 4B.) 



313. 



ic> The use of these organs is by no means confined to one 

 function; but, as in plants, and even in the lower animals, 

 the same organ can perform two apparently opposite func- 

 tions; so these slits appear to be destined as well for the re- 

 ception and preparation of gaseous matters, as for exhalation. 

 The former of these functions seems to be established by the 

 facts, that leaves absorb more powerfully with their under 

 than with their upper surface, and that the slits are more nu- 

 merous in juicy plants, which are nourished more by the sur- 

 faces of the leaves than by the roots. And that the slits ex- 

 hale, and even serve for evaporation, we learn from the expe- 

 riments of Treviranus, in which plates of glass^ fixed to the 



