March, i894. ADAPTATION IN LIVERWORTS. 195 



each with its own type of structure, whereby the same end is 

 attained. 



It must, however, be remembered that in the Muscineae water is 

 absorbed by tlie leaves, and in those which have a soft stem by the 

 whole surface of the plant. 



(i.) First, there are leaves which are simple in shape, but are cut 

 up, and form a more or less spongiose mass. In Ptilidium ciliare and 

 others the main part of the leaf is a cellular plate, but the border is 

 divided into a large number of hair-like cell rows. In species of 

 Trichocolea, however, there is scarcely any cellular plate at all, the leaf 

 being cut down to its base, and resembling a densely-branched twig 

 (Fig. i). The effect of this is very well seen when the British species 

 [T. tomentella) is gathered in damp weather, as it can then be wrung 

 like a sponge. 



The following types depend upon the alteration of the form of the 

 leaf. 



Fig I. Part of leaf of TricJiocoka tomentella. 



Fig. 2. FruUania dilatata ; a. auricle or water sac— 5'. amphigastrium. 



Fig. 3. L,ea.i oi Physiothim gigantenm ; ff. auricle. 



(2.) In species of Lcjennia, Radiila, etc., the lower lobe of the 

 bilobed leaf does not lie in the plane of the upper lobe, but is turned 

 forward, so that the lobes are apposed. The advantage here lies in the 

 increased capillary surface, while in some species a simple sac is pro- 

 duced by the depression of the base of the leaf. 



(3.) In the British species oi FruUania a considerable advance upon 

 the foregoing is found in the bending and hollowing out of the lower 

 lobe, which forms a cup-shaped sac, with a wide mouth opening 

 posteriorly (Fig. 2). In a foreign species (F. cornis:era) there are two 

 sacs to each leaf, and the mouth is narrower than the basal part. It 

 is interesting here to note that such sacs are not confined to the foliose 

 forms, since in Metgeriasaccata, one of the thallose Jungermannieae, pro- 

 jections are developed at the edge of the thallus, which bend round 

 and form sacs comparable with those of FniUania. 



(4.) The last type presents a highly-organised water-sac, or 

 auricle, with its mouth towards the middle or base, and protected by 

 a valve. This sac, which is comparable in structure and develop- 

 ment with the bladders of Utviculavia, is found in species of Colura dnid 

 Physioiiuvi. In Physiotium gigantenm (Fig. 3), a plant inhabiting the dry 

 mountain regions of India and Ceylon, the sac is subulate, and has a 



