SPECIAL MORPHOLOGY: MOSSES. 



187 



themselves above and below on the leaf-cells ; or, finally, it is com- 

 posed of an actual vascular bundle, that is, a large bundle of the 

 above-described (liber?) cells, enclosing elongated wide thin-walled 

 cells (vessels), which are either interposed between the two halves 

 of the leaf, consisting of a single layer, as in Catharinea, or received 

 between the two layers forming the leaf as in Potytrichum. The 

 seta consists of similar elements to the stem, only that here the 

 cells are generally thinner and longer. The cortical cells of the 

 seta, the epidermal cells of the theca and of the operculum, the 

 cells of the peristoma, as well as frequently the cells of the ad- 

 hering fibres, have the cell-walls varying in colour from a light to 

 a dark brownish yellow. The cells of the peristoma generally ex- 

 hibit irregular wart-like thickenings upon their walls, which often 

 appear so prominent, that in the case of Bryum ccespititium, for 

 instance, the points of the dentations seem to be narrowly and 

 deeply notched at the sides. 



It is further remarkable, that the epidermis appears most per- 

 fectly developed on the collum and the apophysis, exhibiting 

 stomates here. There is generally also a small quantity of loose 

 spongy cellular tissue beneath it. 



Simple as the structure of Mosses is, we are very deficient in exact 

 observations regarding many particulars. * Thus the little stem of Bux~ 

 baumia aphylla exhibits much that is interesting, as, for instance, the in- 

 dication of a reticular thickening of the cell-wall in the medulla. The 

 leaves of Mosses and their nerves, also merit more thorough investigation 

 than they have hitherto received. In Catharinea undulata (fig. 139.) 

 the leaf, as in most other Mosses, with the exception of the species of 

 Polytrichum, consists only of one layer of cells (b). The central nerve 

 (c), however, which supports the affixed lamellae (#), consists of an upper 

 and lower epidermis, between which a true vascular bundle is interposed. 

 This (see fig. 140.) consists of liber-cells (b d), enclosing between them 



HO 



* Structure of the Seta in Funaria hygrometrica, by E. Lankestcr, in Ann. of Nat. 

 Hist., by Jardine, Hooker, and Taylor, Feb. 1840, p 361. 



139 Section through the central part of a leaf of Catharinea undulata. a, On the rnid- 

 nerve (c) are erect longitudinal lamellae; b, leaf-cells. The mid-nerve consists of very 

 much thickened liber-like cells, and thin-walled wider ones enclosed by it. 



140 Catharinea unduJata. Section, lengthwise, through the mid-nerve of the leaf. 



