Cavers : Some Points in the Biology of Hcpa/icce. 209 



Cyatliodinni the thallus has a very simple structure, consisting- 

 only of two horizontal layers of cells, united by vertical plates 

 whicli form the partitions between the larg-e air-chambers ; each 

 chamber opens by a sing'Ie wide pore in the roofin*;' layer, whilst 

 the lower layer of cells bears a few smooth-walled root-hairs. 

 In both Dwnortiera and Cyaihodium the ventral scales are g^really 

 reduced, each scale consistinj^ of a simple or branched row of cells. 

 On the other hand, the g^enera Targionia, Preissia, and 

 Reboulia, each represented in Britain by a sing-le species, are 

 chiefly found in exposed situations, where the plants are 

 liable to become dried up at times. In these forms the thallus 

 is capable of resisting- the effects of continued droug-ht, often 

 becoming- rolled up in the same way as Riccia nigreLla, and 

 reviving- when water is ag-ain supplied. The ventral scales are 

 larg-e and deeply coloured, usually extending on to the lower 

 surface of the lateral wing's or even reaching- the marg"ins of the 

 thallus. In Targionia and Preissia the chambers are relatively 

 narrow and deep, the 

 epidermis consists of 

 larg^e cells which con- 

 t a i n 1 i t 1 1 e o r no 

 chlorophyll, the g^reen 

 filaments are well 

 developed, and the 

 pores are small. In 

 Targionia each of the 

 g-reen filaments in the 

 central portion of 

 the chamber ends in 

 a larg-e g-lobular cell 

 containing- very few 

 chloroplasts ; these 

 swollen, colourless 

 cells doubtless serve 

 to screen the underly- 



r^ \trr 



Fig. 7. — Targionia tiypophylla. The upper fig-ure 

 represents a transverse section of the thalUis, x 30; 

 the lower a single air-chamber, X 180. V. S., ventral 

 scales ; adv. br. , an adventitious branch. 



compact tissue of the midrib is traversed by numerous very long- 

 and narrow fibrous cells, having; thick brown-coloured walls 



■* Fruiting specimens of" Preissia coinmuiata and ottier interesting- 

 Hepaticae, collected in various Yorkshire localities, were g-enerously pre- 

 sented to the writer by Mr. W. Ingham, B.A., York. The writer has 

 collected large quantities of Reboulia hemisphen'ca on Malham Moor, near 

 Skipton. 



iq<V, June 



