88 
Cavers. — On the Structure and 
is its peculiar fragrant odour, which resembles that of oil of bergamot, and 
becomes very pronounced when the thallus is crushed. 
The thallus is differentiated into a thicker median cylindrical portion 
(midrib) and a much thinner portion (lamina) on either side of it. This 
is more conspicuous on the lower than on the upper surface, since the 
midrib projects strongly below. On its lower surface the midrib bears 
two rows of scales, one row on either side of the middle line, together with 
numerous tufts of rhizoids, which are also arranged in two rows immediately 
outside of the scales and pass downwards into the soil. 
The ventral scales are relatively small, being confined to the sides 
of the midrib and hardly reaching the lower surface of the lamina. Each 
scale (Plate VI, Fig. 1 6 ) is made up of an oval or kidney-shaped free 
anterior portion, red or violet in colour, and a colourless posterior portion 
which has the form of a long narrow band, attached by its outer edge to 
the side of the midrib, whilst its inner edge reaches and is parallel with 
the middle line of the thallus and overlaps the hinder portion of the 
next scale in front. On tracing the scales forwards and removing them 
one by one, it is found that the coloured appendages are, at the anterior 
margin of the thallus, curved upwards and backwards so as to cover the 
apical growing-point, which lies in the notch observed at the end of each 
branch. As seen from above, this notch appears to be largely filled 
up by the overlapping appendages, which serve to protect the young 
tissues of the growing-point (Plate VI, Fig. i). 
When the scales have been removed the lower surface of the midrib 
shows a median furrow, containing a strand of rhizoids which do not, like 
those arising in tufts from the sides of the midrib, at once grow downwards 
into the soil, but pass backwards in a compact bundle. 
Apical Growing-point. 
The growing-point of each branch is occupied by a transverse row 
of actively-dividing cells, lying nearly in the plane of the ventral surface 
of the thallus. These initial-cells are wedge-shaped, appearing oblong 
and rectangular in surface view, triangular in a longitudinal vertical section 
through the growing-point (cf. Plate VI, Figs. 8, 9, 18). From each initial- 
cell segments are cut off by walls parallel with its upper, lower, and lateral 
walls. The growth of these segments is less rapid in the region immediately 
behind the apex than it is on either side, so that the apex itself comes 
to occupy a deep notch. Whilst a large portion of the tissue formed from 
the segments consists of cells that remain closely packed together, the 
superficial cells of both the dorsal and the ventral segments show 
a specialized form of growth, the former giving rise to the dorsal layer 
of air-chambers, and the latter to the ventral scales and the rhizoids. 
