SECT. I 



MORPHOLOGY 



165 



tlie formation of an epidermis distinct from the fundamental tissue is 

 exceptional. In the thallus of the Marehantieae and on the spore 

 capsules of the Anthocerotaceae and Bryineae, the external layer of 

 cells become more or less sharply defined from the underlying tissues. 

 In the Marehantieae (Fig. 170) this outer laj^er is pierced by openings 

 which have been termed breathing-pores, but these have a different 

 origin from the stomata of higher plants. The air-chamber arises as 

 a small intercellular space between eight cells arranged in a cube 

 which has been derived from a single peripheral cell of the thallus. 

 The intercellular space 

 widens into the air-chamber 

 while the four cells above 

 it become divided to form 

 several tiers in order to 

 give rise to the air -pore 

 opening to the exterior. 

 This when mature has the 

 Ijarrel - shaped appearance 

 represented in Fig. 160 

 A and B (i^i). In the 

 Anthocerotaceae and 

 Bryineae, on the other 

 hand, stomata similar in 

 structure to those of the 

 Pteridophytes and Phaner- 

 ogams are found in the 

 outer cell layer of the spore 

 capsules. It would seem, 

 however, that these stomata 

 of the Bryineae are prob- 

 ably not homologous with 

 those of higher plants. It 



r-~\ 



Fk; 



, 171. — Transverse section of the stem of Mnium undu- 

 lata III. I, Conducting-bundle ; c, cortex ; e, peripheral 

 cell layer of cortex ; /, part of leaf; r, rliizoids. (x 90.) 



is more reasonable to regard them as merely analogous formations, 

 such as from internal causes so often occur in the evolution of 

 organs. In the stems of many of the Bryineae there is also 

 developed a simple form of conducting bundle (Fig. 171); 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 Bryophytes may still be included, just as they 

 were originally in 1813 by Auguste Pyrame De Candolle 

 C^-), in his classification of the vegetable kingdom according to 

 the natural system, with the other lower Cryptogams in the class 

 of CELLULAR PLANTS, as distinguished from the VASCULAR PLANTS, 

 or Pteridophytes and Phanerogams. A separation of the tissues 

 into the three systems of epidermal, fundamental, and vascular 

 tissues occurs for the first time in the vascular plants associated 



