2 1 2 Cavers : Some Points in the Biology of HepaticcB. 



the stomates of higher plants. Of the British Marchantiales^ 

 Targionia hypophylla is the most markedly xerophytic, the 

 faculty of resisting- drought being more highly developed in 

 this form than in Preissia and Reboulia, which, though showing 

 certain adaptations for a xerophytic habitat, are intermediate in 

 this respect between the thoroughly xerophytic Ta?giotiia and 

 the forms which are in nature seldom exposed to the risk of 

 desiccation, e.g., Marchantia. 



Out of about 230 species of Hepaticae occurring in Britain i8 

 belong to the Marchantiales, 16 to the thalloid Jungermanniales, 

 and the remainder, nearly 200, to the foliose Jungermanniales. 

 When we take into account the Hepaticae of the whole world, 

 the disproportion between the thalloid and the foliose forms 

 becomes even more striking, for of about 4,500 species only 



Fig. 10.— Reboulia hemispherica. I., The whole thallus in transverse 

 section, X 15 ; II., part of the epidermis as seen from above, showing 

 three pores, x 40; III., a ventral scale, with a single pointed appendage 



(app.), X 40, 



about 500 are thalloid, the remaining species all belonging to 

 the foliose Jungermanniales. The single genus Plagiochila 

 contains more species than the whole of the thalloid genera 

 together, whilst FriilLania comes second with over 300 species. 

 The foliose Jungermanniales show endless variety in the modi- 

 fication of the leaves. In most cases these are flattened, either con- 

 sisting of a single rounded lobe (e.g., Nardia), or deeply divided 

 into two lobes which may be either about equal in size (e.g., 

 Cephalozid) or markedly unequal ; in the latter case the upper 

 lobe is usually larger than the lower, but the reverse holds good 

 in some forms (e.g., Scapanid) The margin of the leaf is often 

 fringed with fine outgrowths (e.g., Ptilidiinn ciliare), or the 

 leaf may be cut into numerous lobes (e.g., species of Lepidozid) ; 

 from these forms there are various transitional stages to those 



Naturalist 



