Xerophytic Adaptations of Bryophytes. 189 
though neither of them can be considered as truly xerophilous. 
Wall forms of other plants often have thicker leaves than the forms 
of moister habitats ( Barbula unguiculata). 
( b ) Succulent Thallus. The thallus of the Marchantiales, 
Anthocerotales and thalloid Jungermanniales is usually thicker when 
the plants are growing in drier places than their normal habitats. 
This is noticeable in the form crnssior of Aneura pinguis, especially 
so in the extreme form growing in patches on some sand-dunes (14), 
and the same thing is true for the drier ground forms of Riccia, 
Conocephalum, ( Fegatella ), Marchantia , etc. In M. polymorpha there 
are two extreme forms, the var. aquatica , actually growing in the 
water, and var. alpestris occurring on alpine soil; the latter has a 
thicker thallus than the type whilst the former has a thinner one. 
The structure of the thallus in the Marchantiales also has a 
fairly direct relationship to habitat. Air-chambers containing 
assimilatory tissue and communicating with the exterior by small 
pores are present in many of these, but are absent in plants which are 
constantly found in wet situations (4), e.g., Dumortiera, Marcluintia 
polymorpha v. aquatica, and Monoclea. The air-chambers give a supply 
of air to the plant when covered with water, and during exposure 
their walls protect the assimilating tissue from undue evaporation. 
Targionia is one of the best examples of a xerophytic member of 
the Marchantiales combining many of the devices mentioned. 
Conclusion. 
Some of the xerophytic adaptations found in bryophytes also 
occur in the vascular plants, but many are peculiar to the group. 
It must be emphasised that many of the plants mentioned as 
exhibiting “ xerophytic ” devices are not xerophytes; the devices 
simply enable the plant to live under less moist conditions than if 
they were absent. 
In conclusion the writer wishes to acknowledge the kind help 
and valuable criticism given to him by Mr. Tansley and Dr. Cavers 
during the preparation of this paper. 
