67 



cular cells are entirely wanting, on which subject I have re- 

 cently offered some remarks*. It appears as if these aper- 

 tures served to convey the moisture of the air into the interior 

 of the tissue of these plants ; theRiccice grow under similar cir- 

 cumstances as the Marchantice ; at times they have too much 

 moisture, at others too little, and these arrangements might 

 in both cases aid in regulating the proportions. Besides, the 

 vesicular projections on the surface of the Riccite do not always 

 open, a phenomenon quite similar to that of the occurrence 

 of the holes so frequently mentioned on the sides of the cells 

 of some species of Sphagnum, on which subject we shall sub- 

 sequently have occasion to treat more fully. 



Although the cellular tissue of the Riccice is very compact, 

 exactly as in the Marchantice, so that only very rarely inter- 

 cellular passages occur between the cells, yet the substance of 

 the frond, when it occurs but moderately, is abundantly tra- 

 versed by air-receptacles. They arise from a gradual disinte- 

 gration of the cells, are often present only here and there in 

 the frond, and frequently quite irregular, and in the species 

 growing in water they are found to exist constantly. When 

 the frond is thick several series of such air-cavities originate 

 one above the other as in the gibbosity of Lemna gibba. 



All the species of Riccice which have a more fleshy frond 

 present in the centre a still denser layer, consisting of hori- 

 zontally elongated cells. In several species the under surface 

 is moreover covered with a coloured cuticle, which consists of 

 a single or at times even of numerous layers of small coloured 

 cells ; in old age this membrane frequently falls off of itself, 

 and produces, by mere shining through, a colouring of the 

 surface. This coloured layer, says the author, is chiefly the 

 cause of all the red and violet colouring of the Riccia y with 

 the exception of the margin, which in some cases is inde- 

 pendently coloured. Where the margin is involute this co- 

 loured under side occurs above. 



In most species of Ricciae not a trace is found of a vein in 

 the leaves ; in some however bundles of pleurenchymatous 

 cells are gradually formed. These traverse the frond longi- 

 tudinally and send out ramifications on both sides; they are 

 not met with in any species which is not provided with air- 



* Ueber einige Eigenthiimlichkeiten in der Epidermis verschiedenen Or- 

 chideen. Wiegmann's Archiv. 1837, p. 423. 



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