288 



STRUCTURE OF THE BRYALES 



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

 meters in height, but prostrate forms creep over the ground 

 indefinitely. 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, 

 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 



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. 



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 

 reservoirs (Fig. 206, D) . During drought, this delicate apparatus 

 is protected by the coiling of the leaves into needle-like rolls 

 which result in the exposure only of the thickened epidermal cells. 



