176 Cavers: Some Points in the Biology of Hepaticce. 



spring tufts of wide smooth-walled root-hairs which pass 

 straight downwards into the soil and often have branched 

 ends. Besides these root-hairs, which serve to attach the plant 

 and to absorb water, we find narrower ones which spring in small 

 tufts from the bases of the ventral scales and which pass back- 

 wards, lying in a groove on the lower surface of the midrib a.id 

 covered bv the overlapping scales. Each of these narrow root- 

 hairs shows on the internal surface of the cell-wall a series of 

 peg-like ingrowths, and it would appear that root-hairs of this 

 tvpe serve both to conduct and to store up water, which may be 

 retained between the hairs as well as within them. 



It is in the higher Marchantiales, including Fegiitella and 

 its allies, that the thallus reaches the highest type of internal 

 differentiation. The different forms belonging here, though all 

 more or less polymorphic, varying in structure as well as in 

 external habit when growing in different surroundings, are 

 readily distinguished on examining sections of the thallus. Each 

 genus shows its own characteristic features which remain con- 

 stant and enable us to identify the plant even in the absence of 

 fertile specimens. Confining our attention to the British forms, 

 we find that in Marchnntia , Lunularia, Preissia, and Targionia 

 the structure of the thallus is similar in ground-plan to that 

 found in Fegatella, the tissue being differentiated into (1) an 

 epidermal layer, one cell thick, forming a roof over (2) a single 

 series of air-chambers, each opening by a pore in the epidermis 

 and containing numerous green filaments which arise from the 

 floor ; (3) a compact zone of colourless cells, underlying the 

 air-chambers and forming the greater part of the midrib hut 

 becoming reduced to a thin layer in the lateral wings ; (4) root- 

 hairs and scales arising from the lower surface of the midrib and 

 showing the same arrangement as in Fegatella. In Ltinularia 

 and Targionia the pores are of the same type as in Fegatella, 

 each pore being surrounded by several concentric rings of cells 

 which all lie in nearly the same plane, but in Marchantia and 

 Preissia the pores are 'barrel-shaped,' each being bounded by 

 four or five superposed rings of cells forming a hollow cylinder 

 (Figs. 8, 9). In all these forms the compact tissue contains oil- 

 bodies, which are large and spherical or ovoid in form and occur 

 singly in the cells. In Preissia and Targionia there is almost 

 invariably a well-developed mycorhizal zone in the midrib, the 

 cells being often nearly filled by the branching- fungal hyphai, 

 which bear in some cases large vesicles. 

 {To be continued.) 



Naturalist, 



