Cavers : Some Points in the Biology of Hepnticce. 



and creeps along- the soil, and an ascending- portion with broad 

 green wings, e.g., Pallavicinia Lyellii. The wings often become 

 folded or divided into lobes, or repeatedly forked and spread out 

 like a fan. From such forms there is an almost insensible 

 passage into the strictly foliose type (3), that of the leafy 

 Jungermanniales, comprising the great majority of the species 

 oi Hepaticce. Here we find a slender stem, bearing a row of 



leaves on either 

 -^ side, and often also 



a third row on the 

 lower surface. 



In Riccia and 

 its allies the struc- 

 ture of the thallus 

 is fairlv simple. In 

 the common A'. 

 g I ii u c a ,"■> o ft e n 

 f o u n d f o r m i n g 

 rosettes on damp 

 soil, the branches 

 ic<i the thallus are 

 narrow and strap- 

 shaped, showing a 

 median longitudinal 

 groove on the upper 

 surface, which is 

 deepest towards the 

 notch in which lies 

 the growing-point. In sections, the thallus is seen to consist of 

 a compact colourless lower zone and a filamentous upper zone, 

 each of the green filaments consisting of a single row of cells ; 

 between the filaments there are deep, narrow air-spaces (Fig-. 2). 

 The uppermost cell oi each filament is larger than the rest, and 

 forms, with the corresponding cells oi the neighbouring fila- 

 nients, a continuous epidermal layer. These epidermal cells 

 contain colourless sap, with few chloroplasts or none at all ; 

 they doubtless serve to protect the underlying green cells from 

 excessive illumination, besides partially closing the air-spaces 

 and thus checking evaporation and at the same time preventing 

 the entrance of water into these spaces. The lower surface of 



* For an excellent supply of living- plants belonging- to this species, the 

 writer is indebted to the kindness of Mr. Geo. Webster, York. Material of 

 other British species was also kindly sent by Mr. W. H. Pearson. 



NaHirali-.t. 



Fig. 2. —Riccia glauca. Part of a transverse' section uf 

 thallus, with a fertilised archcgoniimi containing a yoLin^ 

 capsule. X 50. 



