OP INDIA AND CEYLON. 
191 
the thalliis of “ root ” nature seen in the remaining genera ; 
in some the root is a mere creeping thread, in others a 
creeping closely attached ribbon, or discoid body (Hydro- 
bryum spp.), in others again a drifting thread or ribbon 
attached to the rock only or principally at the base, or a 
fucoid organ, or even a cup-like body, as in Griffithella."^" 
Within the same species the form of the thallus is very 
variable in detail (the polymorphism reaches an extra- 
ordinary degree in GrifBthella*), but examination shows 
that the main morphological features of it are very constant, 
and therefore of high classificatory value, all the Dicræasf 
have a more or less free drifting root thallus, exogenously 
branched, with secondary shoots on the upper margins and 
in the angles ; all the FarmeriasJ have closely creeping 
endogenously branched thalli, with the secondary axes in 
the basiscopic angles, and so on. The degree of attachment 
to the rock is variable, as is well illustrated in Griffithella ; 
so also is the degree of lobing or branching, as seen in species 
of Dicræa and Hydrobryum, and in the shoot thalli of Lawia. 
The size is also very variable. Many thalli alter very much 
in form as they approach the flowering season, and still more, 
as might be expected in water plants, after they have become 
exposed to the air in the dry season. The herbarium speci- 
mens of these plants are in very many cases merely examples 
of these exposed dead thalli, and owing to the great difficulty 
(practical impossibility in the case of such closely attached 
thalli as those of some Lawias and Hydrobryums) of detach- 
ing the thalli from the rocks are usually of the most 
fragmentary description. In the vegetative season the 
thallus is usually beset with secondary shoots almost to the 
extreme tips, but very often many of these do not form 
flowers, and when the flowers are ripening for anthesis the 
thallus often breaks away , except in the florif erous part. This 
is especially marked in the Dicræas, where in D, elongata the 
Cf. Plates XXV., XXVI. of subsequent paper, 
t Cf. Plates XVIII.-XXIV. of subsequent paper, 
f Cf. Plates XXXVIII. XL. of subsequent paper. 
