SECT. I 



MORPHOLOGY 



153 



more elongated cells function as a conducting tissue. The relatively 

 highest degree of internal development found in the Algae is attained 

 by the Laminariae. In their stem-like axis, which may have a consider- 

 able thickness, the external tissues frequently contain canals filled 

 with mucilaginous matter ; while internally are found rows of cells 

 resembling sieve-tubes ( 108 ). The axes themselves grow in thickness 

 through the continuous division of the cells of one or more of the 

 outer cell layers. A kind of cortical tissue is formed as a result 

 of this growth which exhibits concentric layers ; the innermost 

 cells gradually elongate 

 and pass over into the 

 so-called medulla. In the 

 larger Fungi internal differ- 

 entiation is the result of 

 the more or less intimate 

 union of the intertwining 

 hyphse. In extreme cases 

 the hyphae forming the f \ 

 body of the Fungus may 

 be so closely woven to- 

 gether as to give, in a 

 cross-section, the impres- 

 sion of a parenchymatous 

 tissue (Figs. 106, 107), in 

 which, on the subsequent 

 thickening of the cell walls, 

 the pits inadjoininghyphae 

 coincide. In the fructifica- 

 tions of many of the Hy- 

 menomycetes and Gastero- 

 mycetes, someof thelonger 

 and more swollen hyphae 

 contain a homogeneous or 



cloudy, highly refractive, and, in some instances, coloured substance, 

 and appear, accordingly, to serve as a special tissue for the purpose of 

 conduction. A marked advance in the differentiation into different 

 tissue systems is first apparent in the Bryophytes, but even in them 

 the formation of an epidermis distinct from the fundamental tissue is 

 exceptional. In the thallus of the Marchantieae 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 Marchantieae (Fig. 160) this outer layer is pierced by openings 

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

 origin from the stomata of higher plants. They are rather, as 

 LEITGEB ( 144 ) has shown, openings into cavities, which have arisen 

 through the overarching of certain portions of the surface by other 



Flu. 161. Transverse section of tlie stem of Mnium undu- 

 latum. I, Conducting-bundle ; c, cortex ; e, peripheral 

 cell layer of cortex ; /, part of leaf ; r, rhizoids. ( X 90.) 



