CHAPTER XVIII. 

 VEGETATIVE STRUCTURE OF THE BRYOPHYTA. 



PRINCIPAL MATERIALS USED. 



Plants of Mnium y or Bryum; fresh, or dried, herbarium material, properly 



softened, will do. 

 Plants of Spliagnum acutifolium, or other species ; fresh, or dried plants, 



properly softened. 

 Marchantia polymorpha, or other species ; fresh. 



PRINCIPAL REAGENTS USED. 

 Glycerine-gum Sulphuric acid. 



Stem of Mnium. Hitherto we have studied only the structure 

 of the stem, root and leaf in Vascular Plants ; we turn now to the 

 stem and leaves of Mosses, from which vessels are absent. We 

 commence with a case in which the differentiation of tissues 

 appears already somewhat advanced ; with Mnium undulatum. 

 We first of all prepare delicate cross- sections through the stem 

 (Fig. 90). In the midst of the stem appears an axial cylinder, 

 composed of narrow, thin-walled cells, which we can only conceive 

 as a very simple conducting bundle (t). Its cells are distinguished 

 from their surroundings by the yellow- brown coloration of their 

 walls. To this conducting bundle adjoin the wider cells of the 

 cortex, with greenish -yellow walls, living contents and chlorophyll 

 (c). The innermost layer of this cortex is marked by its greater 

 resistance to sulphuric acid, without being in any way sharply 

 distinguishable as a protective sheath or endodermis. At first the 

 cortical cells enlarge somewhat in passing from the interior out- 

 wards ; at the periphery they become rapidly narrower and thicker 

 walled, and pass over at length, without special limit, into a uni- 

 or bi-lamellar, narrow-cavitied, strongly-thickened epidermis, At 

 two or three places, or sometimes only in one, the outer cell layer 



(228) 



