94 
Cavers. — On the Structure and 
mucilage-hair, at first terminal but now standing in the axil of (2) the 
appendage, which is inserted by a narrow neck on (3) the long and narrow 
basal portion of the scale. 
Rhizoids. 
Each of the rhizoids springing from the lower surface of the thallus is 
formed by the outgrowth of a single superficial cell, and remains throughout 
undivided, though often reaching a length of an inch or more. The rhizoids 
are of two kinds, some being smooth-walled, whilst in others the wall shows 
numerous peg-like thickenings which project inwards and are arranged 
in a fairly definite spiral line. The smooth-walled rhizoids spring chiefly 
from the sides of the midrib, immediately outside the ventral scales, and 
pass straight down into the substratum, where their ends often become 
branched (PI. VI, Fig. 25). The tuberculate rhizoids arise in small bundles, 
each bundle being borne in the axil of a ventral scale, and these unite to 
form the compact bundle which occupies the median groove on the lower 
surface of the midrib. The tuberculate rhizoids generally end freely in the 
median bundle and do not become branched. 
The superficial cell which will produce a rhizoid is recognizable at an 
early stage on account of its large size and densely granular contents. 
This cell projects from the surface and grows enormously in length, but 
no cell-divisions occur, the rhizoid being simply an elongated cell (PI. VI, 
Fig. 26). The young rhizoid shows strictly apical growth in length, the 
protoplasm and nucleus being found near the growing tip, whilst further 
back the rhizoid contains only cell-sap. The plain rhizoids give the 
reactions of cellulose, but in the tuberculate ones the tubercles themselves 
become altered in composition, and on treating the rhizoid with sulphuric 
acid the tubercles alone remain unaltered after the rest of the cell-wall has 
been dissolved. 
Leitgeb (1881, p. 20) suggested that the tuberculate rhizoids of the 
Marchantiaceae not only shared with the smooth ones the function of 
attaching the plant to the soil and of absorbing water, but might also have 
a third function, namely, that of strengthening the thallus. As pointed out 
by Kamerling (1897, p. 12), however, the turgor of the cells composing the 
ventral tissue of the thallus is quite sufficient to maintain the rigidity 
of the latter, which remains unaltered when the median bundle of tuber- 
culate rhizoids is cut through at various points. Kny (1890, p. 371) believed 
that the tubercles would serve to prevent the walls of the rhizoids from 
sinking in through lateral pressure or lack of water; Haberlandt (1896, 
p. 196), that they increase the area for absorption, these ingrowths causing 
the protoplasmic lining of the cell-wall to become spread out. There is 
little to be said in favour of either of these conjectures, for (1) neither the 
plain nor the tuberculate rhizoids collapse to any great extent when plants 
