HEPATICJE. 343 



or rows of cells ; these are attached to the under surface of 

 the thallus, or to the side of the stem, and serve to ■support 

 and fix the plant, as well as to absorb nutritious substances 

 for its sustenance. 



443. — The tissues of Bryophyta are much more highly 

 developed than in the preceding divisions ; the epidermis is 

 in many cases quite well defined, and here for the first time 

 true stomata make their appearance (paragraph 119,page 91). 

 The greater part of the plant-body is in most cases composed 

 of a well-developed parenchyma, composed of thin-walled 

 cells, which are compacted into a true tissue. There is, 

 moreover, a slight indication of the development of a fibro- 

 vascular system in the elongated bundles of cells which oc- 

 cur in the leaf veins and the axial portions of the stems of 

 some of the species. The cells immediately beneath the epi- 

 dermis are much thickened in some cases, so as to form a 

 strengthening tissue. This may be regarded as a simple 

 kind of sclerenchyma. 



444 — The Bryophytes are usually divided into two classes, 

 the Liverworts {Hejmticm) and the Mosses {Musci). 



, 8 I. Class riEPATicsi. 



445. — In this class of plants, commonly called the Liver- 

 worts, the plant-body is for the most jjart either a true 

 thallus Or a thalloid structure. Even when there is a differ- 

 entiation into stem and leaves, it still retains some of the 

 peculiarities of the thallus ; thus iu most cases the plant- 

 body has two distinct and well-marked surfaces, an upper or 

 dorsal, and an under or ventral one, the latter bearing, for 

 the most part, the rhizoids, by means of which the plant is 

 fixed to the ground. Growth is always from an apical cell. 



446. — The tissues of the Liverworts are quite simple, and 

 even in the leaf-bearing kinds there is but little differentia- 

 tion ; the leaves, when present, have no midrib or other veins, 

 but consist of a simple plate of cells. The mode of branch-, 

 ing is dichotomous in the lower species — i.e., those with a 

 thallus or thalloid plant-body — while in those which have 

 stem and leaves it is lateral and monopodial. 



