Biology of Fegatella conic a. 
89 
Apical Branching. 
Under normal conditions branching only takes place at the apex 
of the thallus. In autumn, usually during October, resting-branches are 
laid down at the apex of each lobe of the thallus. During the winter 
months vegetative growth is practically at a standstill, but towards the 
end of January or beginning of February the resting-branches resume 
their growth. When branching is about to occur the initial-cells grow 
in breadth and undergo repeated divisions by vertical walls, thus becoming 
increased in number, whilst the row becomes laterally extended. The 
central cells for some time grow more rapidly than those on either side, 
and form a projecting lobe, but their growth soon ceases, and the cells 
on each side of the lobe, continuing to grow and divide, form the apical 
growing-points of the two branches (Plate VI, Fig. 9). This process is 
repeated, so that four growing-points occupy the end of each of the club- 
shaped outgrowths formed in autumn. These outgrowths, one of which is 
nearly always to be found at the end of each of the thallus-lobes, remain 
dormant during the winter, the four growing-points being protected by 
the coloured scale-appendages (Plate VI, Fig. 2). In early spring growth 
is resumed, and each growing-point may then give rise to a broad expanded 
lobe, or one of them may have its growth in length arrested by the 
development of a male or female receptacle (Plate VI, Figs. 3-7). The 
thallus acquires a more or less jointed appearance, the earliest part of each 
season’s growth giving rise to a narrow stalk-like portion which then 
spreads out to form the broad lobes of the thallus. Occasionally the 
branching deviates from the type just described, owing to suppression or 
repetition of some of the divisions that normally occur. When plants are 
cultivated indoors the branches remain narrow and often cylindrical, and 
grow upwards instead of horizontally. This upward growth is especially 
marked when the plants are cultivated in a deep vessel illuminated from 
above. Plants kept in darkness present a somewhat similar mode of 
growth, the etiolated branches being long, narrow, and cylindrical. 
Air-chambers. 
Immediately behind the growing-point, each dorsal segment of the 
initial-cells becomes divided into an inner cell, which contributes to the 
formation of the compact tissue of the thallus, and an outer cell which 
gives rise to part of the spongy tissue. The outer cells soon begin to 
grow out and to become separated from each other so as to leave narrow 
air-spaces between neighbouring cells. As the tissues behind the growing- 
point become extended in area in consequence of active growth these 
spaces become widened, whilst the superficial cells grow up and remain 
attached laterally so as to form a series of vertical plates. The uppermost 
