92 
Cavers . — On the Structure and 
the bodies consisted of resin or wax. Kuster (1894) has recently confirmed 
and extended the results of Pfeffer’s work by his exhaustive study of the 
oil-bodies. He finds that each oil-body consists of a ground-mass in 
which the oil and other substances are embedded. It would appear that 
in the living cell there is no special envelope around the oil-body, and 
that the membrane which becomes visible on treatment with alcohol is 
simply an artifact due to the action of this reagent on the substance 
of the ground-mass. 
The oil-bodies are distributed throughout the compact ventral tissue, 
both in the thallus and in the sexual receptacles ; they sometimes occur in 
the walls of the air-chambers, and occasionally in the epidermis. The only 
parts from which they appear to be invariably absent are the rhizoids and 
the sporogonia ; the spores contain numerous small oil-drops, but no oil- 
bodies. It is evidently to the presence of the oil-bodies that the thallus of 
Fegatella owes its characteristic odour, which can no longer be perceived 
when the plants are soaked in alcohol and then washed in water. When 
once the oil-bodies are formed they appear to remain unchanged until the 
death of the cells containing them ; plants may be kept in darkness for 
weeks or even months, and the new parts formed are invariably found 
to contain oil-bodies, whilst those already present in the older parts remain 
unaltered. These bodies can therefore only be regarded as products of 
excretion, but they appear to play an important part in the economy of the 
plant. As shown by Stahl (1888, p. 49), they serve to protect the plant 
against the attacks of snails, which will shun fresh pieces of the thallus of 
Fegatella even when there is no other available food. If, however, pieces 
of the thallus are soaked in alcohol and washed in water, they will be readily 
eaten by snails which reject the fresh pieces. 
Mucilage-sacs. In a transverse section through the thallus (PI. VI, 
Fig. 17), the midrib is seen to contain a number (from one to six) of large 
rounded elements which stand out sharply from the smaller polygonal cells 
surrounding them. Comparison with longitudinal sections shows that these 
rounded cells are arranged in continuous rows which traverse the midrib 
longitudinally and which may be traced forwards to the growing-point. 
These cells contain a highly refractive granular substance, which shows 
a stratified appearance, the layers being concentric with the cell-walls. 
On treatment with water, these cells increase in volume, their contents 
absorbing water and becoming homogeneous and transparent ; the transverse 
walls between the cells of each row at the same time lose their definite 
outlines and become disorganized, so that these organs have generally been 
described as continuous tubes. The cells which will give rise to a row of 
mucilage-sacs are recognizable at an early stage, becoming differentiated just 
behind the growing-point (PL VI, Fig. 18, M. o.). These cells are distinguished 
from their neighbours by their dense contents and the absence of chloroplasts 
