302 



STRUCTURE OF THE BRYALES 



is attained by many of the Bryales. It is noteworthy, however, 

 that the devices for absorption and conduction of water are not 

 as yet sufficiently developed to enable these plants to attain 

 any considerable height. Erect forms rarely exceed a few centi- 

 meters in height, but prostrate forms creep over the ground in- 

 definitely. Some mosses are short lived and many are per- 

 ennial, continuing their apical growth from year to year. Some 

 can live only in the wettest of places or as submerged aquatics, 



\ 



FIG. 206. Structural features of the moss plant; A, cross-section through 

 the basal region of a moss stem, showing rhizoidal outgrowths from epi- 

 dermis, cortical and central conducting tissues. B, leaf of Mnium with mid- 

 vein of elongated conducting cells. C, rhizoids twisted together into root- 

 like strands. D, cross-section of a leaf of Polytrichum, showing the partial 

 folding in of the margins of the leaf to protect the delicate plates of chloro- 

 phyll-bearing cells against drought. 



while others are capable of enduring almost complete desiccation 

 and revive quickly when moistened. An interesting device of 

 service in adapting these plants to periods of drought appears 

 in the leaves of some of the higher forms, as the hair cap moss, 

 Polytrichum. Plates of green cells parallel to the midvein project 

 from the upper surface of the leaves, thus greatly increasing the 

 surface of the leaves for photosynthesis and also serving as water 



