188 



MORPHOLOGY. 



TTT 



long, very much expanded, cylindrical cells (c) (vessels in the simplest 

 form). Robert Brown made the correct observation that in Catharinea the 

 lamellae are only inserted upon the mid-nerve, while in Polytrichum they 

 are found upon the whole surface of the leaf. (See his Miscellaneous 

 Writings.) It does not seem to me that Treviranus (in Linnasa, vol. xv. 

 p. 3. plate 3. fig. 6.) refutes this view, or establishes his own that the 

 whole is to be regarded as a nerve in Polytrichum. The lamellas placed 

 upon the surface of the leaf in Polytrichum exhibit the peculiarity that 

 the lower cells are always thin-walled, while the upper and lateral es- 

 pecially are very much thickened. In P. yucccefolium these upper walls 

 are bent in, so that each lamella shows a furrow upon its free edge. 



Many have contested the view of 

 there being marginal nerves in the 

 leaves of Mosses, but without having 

 examined the subject. In Mnium 

 punctatum, for instance, they are 

 strikingly evident, and formed from 

 thickened cells ranged in layers 

 (fig. 141. A b). The cells of the la- 

 mina of the leaf (a) in this plant like- 

 wise exhibit interesting peculiarities 

 from the manner in which they are 

 ranged together, as may be seen from 

 fig. 141. B, C). In a group* of 

 Mosses consisting of Sphagnum, Oc- 

 toblepharum, Leucobryum, Dicranum 

 glaucum, and Weissia verticillata ? 

 the leaf is essentially composed of 

 two different kinds of cells ; namely, 

 some that are closed, narrow, containing chlorophyll, and others broader. 

 These latter distinctly exhibit thickening layers, either merely as large 

 pores, which subsequently become actual apertures, or, as in Sphagnum, 

 also spiral fibres ; they lie either in the same plane with the green cells 

 {Sphagnum), or they cover the reticular layer of green cells on both 

 surfaces, in layers varying from one to five. Much contention has been 

 kept up, by Meyen especially, concerning the structure of the Sphagnum 

 leaf; but this must be considered to be wholly set at rest by Mohl.f 



The stomata on the capsule of Mosses, however simple the subject 

 may be (there being really not the slightest difference between these and 

 the openings found in the Phanerogamia), have likewise given rise to 

 wonderful discussions ; and botanical mystifiers, instead of treating nature 

 simply, as sound sense dictates, have been pleased to express themselves 

 as follows : "The pores, as peripheric members allied to the spiral ves- 

 sels, although in their structure they cannot be compared in any respect 

 to the true pores of the epidermis (the reason wherefore neither is nor 



* LeucophanecE, according to Hampe. 



f Anatom. Untersuchungen iiber die porosen Zellen von Sphagnum. Tubingen, 1837. 



a, Epidermis of the lower surface ; b and d, liber-like cells ; c, vasculose expanded 

 cells. 



141 Mnium punctatum. A, A section through the margin of the leaf: a, leaf-cells; 



b, marginal nerve. J9, Partition between the leaf-cells in their upper part, very much 

 enlarged. C, A few leaf-cells seen from the surface ; the fine lines separating the 

 cavity of one cell from the other can easily be explained by a comparison with B. 



