274 



STRUCTURE OF THE BRYALES 



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 



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

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

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

 midvein 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 chlorophyll-bearing cells against drought. 



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 pho- 

 tosynthesis and also serving as water reservoirs (Fig. -'<)(), D). 

 During drought, this delicate apparatus is protected by the coil- 

 ing of the leaves into needle-like rolls which result in the expo- 

 sure only of the thickened epidermal cells. 



The sexual organs are developed on the apices of the main or 

 lateral branches and they are protected by modified leaves which 

 are often colored and form a more or less conspicuous bud or 



