222 MOSSES AND FERNS chap. 



thickened, and have a conspicuous cuticle. Within this epi- 

 dermis are closely set, small sclerenchymatous elongated cells, 

 among which are found more or less definite rows of large, 

 thin-walled elements, strongly suggesting the tracheary tissue 

 of the vascular plants, and without much question, true water- 

 conducting structures. From the inner ventral surface there 

 arise numerous parallel, thin, vertical laminae (cl.) composed 

 of green cells. These extend nearly the whole length of the 

 leaves and in section appear as rows of short cells, the outer- 

 most ones being somewhat enlarged. 



The axis of the shoot in the Polytrichaceae shows a decidedly 

 complex structure and many reach a relatively large size. 

 Thus in Dawsonia superba (Figs. 120, 122) it is about 1.5 mm. 

 in diameter, and forms an erect, densely leafy shoot 40 to 50 

 centimetres in height. The cross-section of the shoot in the 

 latter species (Fig. 122) is triangular in outline. Within the 

 firm epidermis there are several layers of somewhat similar, 

 but more compact cells, which like the epiderrnal cells are thick- 

 walled, and dark coloured. This compact hypodermal tissue 

 passes somewhat gradually into a colourless, parenchymatous 

 ground-tissue, which makes up the bulk of the shoot-axis. 

 There is a very conspicuous central cylinder composed of two 

 tissue-elements — small, dark-colored sclerenchyma or fibrous 

 tissue, especially compact toward the centre of the cylinder ; and 

 very much larger, thin-walled cells, appearing almost destitute 

 of protoplasmic contents, and closely resembling the vessels of 

 true vascular plants, and like them, no doubt, true water-con- 

 ducting organs. Traversing the ground tissue are slender 

 strands of elongated cells — leaf-traces, which are structurally 

 like the central cylinder of the shoot, but with the water- 

 conducting cells less conspicuous. Most of the cells in the 

 stem of Dawsonia, except the large tracheary cells of the central 

 cylinder, contain starch, which it is stated by Goebel (8) is not 

 abundant in the tissues of Polytrichum, where its place is taken 

 largely by oil. Starch has been noted in Polyirichum in the 

 outer cells of the stem and in the leaf-traces. 



The leaf-traces, or continuation of the central tissue of the 

 midribs of the leaves, bend down into the stem, and finally 

 unite with the axial cylinder of the latter, in a manner 

 quite analogous to that found in the stems of many vascular 

 plants. 



