i B6 
W. Watson. 
B. WATER-STORAGE METHODS. 
Many of the methods by which water is quickly absorbed and 
then stored up by the plant have been briefly alluded to in the 
previous pages, for the same apparatus often serves a double purpose 
not only forming small water-reservoirs but also preventing evapo¬ 
ration. Every little contrivance for retaining water is taken 
advantage of and a device which is of infinitesimal use in a large 
vascular plant becomes important in these small plants. The 
following are the most important methods by which water is 
stored up. 
(1). Water-Sacs. 
The under-lobes of the leaves of a large number of Hepatic® 
are modified to form water-sacs. These are very definite in shape, 
forming a distinct oval or elliptical bag with a narrow mouth in 
Frullania (Fig. 2, 10), Pleurozia, Polyotus (7). In Colurolejeunea 
the water-sac is a pouch-like body, and sometimes has a valve 
which opens inwards but not outwards. 
Water-sacs are also found in some thalloid liverworts. Some 
of our indigenous hepatics have little pits which retain water, but 
these can scarcely be dignified by the name of water-sacs; however, 
some foreign liverworts (e.g., Metzger ia saccata, Dendroceros foliatus) 
have outgrowths on the thallus which have a similar hood-like 
character to those of Frullania (7). 
Water-sacs may be formed by the two lobes of the leaf being 
laid against each other, so that a small chamber is formed; the 
water trickles down the stem, is collected in the lobes, so that none 
is lost. This is a common method amongst liverworts, and is also 
shown in the moss Fissidetis (Fig. 3, 14). Their best development 
is shown in the Lejeune®— Cololejeunea, Microlejeunea, Drepano- 
lejeunea, Harpalejeuaea, Marchesinia, Lejeunea cavifolia (Fig. 2, 2), 
L. patens, L. Macvicari, etc. They are also present in species of 
Radula, Scapania, Diplophyllum, and Madotheca. This apposition 
of the two lobes is not confined to terrestrial species, but is best 
defined in them, the aquatic plant often having the lobes very widely 
divergent as 1 have found to be the case with Scapania undulata 
when growing in clear springs on Millstone Grit (Fig. 2, 8, 9). In 
Lejeunea cavifolia, the variety planiuscula of wetter places has the 
postical lobes almost obsolete. 
In Fissidens (Fig. 3, 14) the lower part of the leaf is conduplicate 
and partially embraces the stem, so that not only is the evaporating 
surface reduced, but small chambers are formed, thus enabling the 
plant to absorb the water quickly and afterwards retain it. To 
