Cavers . — On the Structure and 
1 14 
throughout the entire year. The importance of systematic outdoor 
observations at all seasons of the year can hardly be over-estimated in the 
case of the Hepaticae and the Mosses ; even in the middle of winter these 
plants yield material which is absolutely necessary to the investigator who 
wishes to follow the successive stages in their life-histories. 
Before the dehiscence of the ripe capsule takes place its seta grows in 
length, owing to simple elongation of its constituent cells, the starch-grains 
originally present in these cells becoming used up in the process. The 
seta finally becomes as long as, or a little longer than, the capsule itself, 
so that the latter is thrust out through the ruptured calyptra and projects 
beyond the opening of the sheath (involucre). It frequently happens that 
the seta becomes broken off at the base, so that the entire capsule falls 
from the receptacle. 
Germination of the Spore. 
As already stated, the ripe spore begins its germination before the 
dehiscence of the capsule takes place, so that on being liberated it encloses 
an ovoid mass of cells, usually five or six in number. At either end of 
the longer axis there is a bluntly conical cell, distinguished from the middle 
cell by containing fewer chloroplasts. When the spore is set free and its 
germination is resumed, one or other of these terminal cells grows out 
to form a colourless rhizoid (PI. VII, Fig. 54), the exospore not showing any 
definite rupture but simply becoming stretched out. It appears that here } 
as in other cases that have been investigated (cf. Zimmermann, 1879 ; 
Heald, 1898), light is the chief factor in determining the point of origin 
of the primary rhizoids, which invariably spring from the end which is 
furthest from the light, whilst the illuminated end grows out to form the 
apex of the young plant. The latter at first appears to grow by a single 
initial cell, but soon the typical transverse row is established, and the 
differentiation of the tissues proceeds as in the adult plant. At first only 
smooth-walled rhizoids are formed, whilst the ventral scales are simpler 
in structure than in the fully-grown thallus, but soon the typical structure 
of the latter is attained with the appearance of tuberculate rhizoids, coloured 
scale-appendages, mucilage-sacs, oil-cells, and well-developed air-chambers. 
The spores will usually germinate promptly on being sown directly after 
the dehiscence of the capsule, but they are not adapted to undergo 
a resting period, and if allowed to become dry they soon perish. In 
darkness they usually remain unaltered, though sometimes cultures in the 
dark show the development of rhizoids from one or both ends of the 
mass. 
