2 10 Cavers: So?ne Points in ^he Biology of Hepaiicce. 



(Figs. 8, 9). In Reboulia the development of the air-chambers 

 begins in the usual manner, but the primary chambers rapidly 

 gTOw in size and become partitioned up by thin plates, so that 

 in vertical sections the thallus is seen to be largely made up of 

 a network of chambers, recalling the structure of Ricciocarpus 

 tiatans (Figs. 10, 11). In the midrib there is a fairly thick zone 

 of compact colourless tissue below the spongy green tissue, but 

 in the thin wing on either side, the compact tissue is reduced to 

 two or three layers of cells or even to a single layer. The large 

 cells of the epidermis are colourless and their outer and vertical 

 walls are strongly thickened, especially at the angles between 

 adjacent cells. It will be seen that Reboulia differs considerably 

 in structure from the remaining British Marchantiales, from 

 which it is therefore readily distinguished when sections of the 

 thallus are examined. The same type of structure is found in 

 several other genera which together form a well-marked group, 

 the ' Operculatse ' of Leitgeb (e.g., Plagiochasma, Gi'imaldia^ 

 Fimhriarid). All of these forms are, like Reboulia hemispherica, 

 typically found in elevated regions, especially on limestone 



rocks, and are 

 adapted to en- 

 dure periodi- 

 cal drying-up, 

 to which they 

 are exposed in 

 nature. In 

 most of these 

 forms, as also 

 in Targionia, 

 the thallus be- 

 comes rolled 

 up when dry, 

 so that the 

 dorsal green 

 tissue comes 



to occupy the interior of a tube, whose outer surface is formed 

 bv the lower side of the thallus, bearing the broad purple scales. 

 In this condition the thallus retains its vitality during long 

 periods of drought, and on being moistened becomes unrolled 

 and resumes its ordinary mode of life. From Mattirolo's 

 interesting observations and experiments on Grimaldia dicho- 

 toma it appears that the roUing-up of the thallus depends on the 

 fact that the cell-walls in the ventral compact tissue are largely 



fubRh. 



Fig. 8. — Preissia commutata. Part of a transverse section of the 

 thallus. A. C, air-chamber; P., a pore; Ep., epidermis ; Sm. rli.. 

 smooth-walled root-hairs; tub. rh., tuberculate root-hairs ; V. S., 

 ventral scales ; Scl., sclerotic fibres of the midrib. X40. 



