232 



XVIII. STRUCTURE OF THE BRYOPHYTA. 



together the cavities of these cells can be readily proved in places 

 where the sections have cut through such pores. Not infrequently, 

 moreover, fungal threads are seen in these cells, which pass with- 

 out hindrance from one cell to the other through the pores. These 

 porous elements of the outer cortex of Sphagnum contain only 

 water or air, and are without living cell-contents. To the plant 

 they serve as a capillary apparatus, by which the water may be 

 carried to a place of need. The plants are devoid of cuticularised 

 parts ; concentrated sulphuric acid immediately dissolves the entire 

 tissue ; comparatively the most resistant are the middle-lamellae, 



. FIG. 92. A, from the leaf of Sphagnum cymbifolium. a, chlorophyll-containing 

 cells ; w, water-cells with thickening ribs and openings (/) ; surface view ( x 300). B, 

 cross-section through the leaf of Sphagnum jimbriatum. C, part of a cross-section 

 through the stem of Sphagnum cymbifolium. c, centre ; sk, sclerenchymatous cortical 

 cells ; w, water-cells with openings and thickening ridges ; e, epidermis (x 120). 



and their junction " seams," of the yellow- brown outer cells of the 

 central cylinder. 



Leaf of Sphagnum. The leaf expansion is ovate, entire, uni- 

 lamellar, and consists, as either surface view shows, of two kinds 

 of elements. The one are narrow, chlorophyll-containing (and 

 therefore also with protoplasm and nucleus) living cells ; and 

 others are dead, filled with air or water, provided with rings or 

 with spiral bands and intermediate open pores. The fact, which 

 must have repeatedly struck us, that dead air- or water- containing 

 cells, so far as they are not strongly thickened, so often need spiral 

 band, rings or network as thickening of their walls, derives explana- 

 tion from the circumstance that these cells are devoid of turgidity, 



