146 



BOTANY 



stomata similar in structure to those of the Pteridophytes and Phanero- 

 gams are found in the outer cell layer at the base of the spore capsules. 

 It would seem, however, that these stomata of the Bryineae are prob- 

 ably not homologous with those of higher plants ; as there is no direct 

 phylogenetic connection between them, and it is more reasonable to 

 regard them as merely analogous formations, such as so often occur 

 in the evolution of organs. The Marchantieae also possess mucilage 

 passages, which arise through the mucilaginous degeneration of single 

 cells or cell rows. Certain of the Marchantieae have also strands of 

 greatly elongated, dark-walled cells. In all Hepaticae there may be 

 found in special cells characteristic oil bodies of an irregular, clustered 

 shape. It is also worthy of note that, although the differentiation 

 of the internal tissues has progressed further in the Marchantieae than " 

 in any other of the Hepaticae, in their external segmentation they 



Fig. 158.— Surface and transverse view of the thallus of Marchantia polymorpha. In A, an air- 

 pore, as seen from above ; in B, as seen in cross-section, (x 240.) 



are surpassed by many others of the same group ; so that here also 

 internal and external differentiation do not keep pace with one another. 

 Among the Musci the Bog-Mosses (Sphagnaceae) are characterised by 

 an external sheathing of porous cells about the stem (Fig. 320). The 

 sheathing cells recall those on many aerial roots (p. 100), and in the 

 Bog-Mosses they also consist of dead cells with porous and spirally 

 thickened walls. By means of this sheath water is drawn up from 

 the ground by capillarity and conveyed to the leaves, throughout which 

 similar porous and dead cells are regularly distributed. In the 

 stems of many of the Bryineae there is also developed a simple form 

 of conducting tissue (Fig. 159); and the many-layered midrib of 

 the single -layered leaf lamina is also traversed by a conducting 

 strand. In spite of their more advanced differentiation, the Bryo- 

 phytes may still be included, just as they were originally in 1813 by 

 De Candolle, in his classification of the vegetable kingdom according 



