26 DIVISION I.—GENERAL MORPHOLOGY. 
to conduct air (Fig.10). The spaces widen most in the centre of the axile strand ; in 
the simplest case a single large axile cavity is formed, and narrow air-spaces join on 
to it on the side of the periphery; in other cases single rows of cells remain in the 
centre of the cavity separated for the most part from the adjacent tissue and therefore 
quickly drying up; here too therefore there is really an air-filled axile cavity ; its 
diameter varies much, but it is always at least half as large as that of the strand, and 
in strong specimens may reach a much larger relative size. 
The wall which encloses the cavity consists in its immediate vicinity of the 
original large axile cells; these form about six irregular layers round the cavity, the 
cells of the outer layers becoming gradually narrower, as was stated above, and it is 
these layers which give rise to the medulla of the fully formed strand in the way which 
will be described presently. Outside of the zone which produces the medulla are the 
numerous layers of the close compact tissue, which ultimately forms the rind of the 
strand. This tissue does not however extend to the surface ; this is occupied by a 
supplementary stratum of about six layers of hyphae with narrow lumina and thick 
gelatinous walls, which have coalesced into homogeneous mucilage, the gelatinous 
felt mentioned above. The hyphae of this tissue run for the most part longitudinally, 
and join with the hyphae of the apical tuft. They also give off spreading branches 
from the surface. From these must be distinguished other spreading branches, also 
provided with gelatinous walls, which spring from the hyphae of the rind beneath the 
gelatinous felt, and pass transversely through it towards the outside. Their number 
and distinction vary in individual specimens, and according to Hartig they are of special 
importance when the Fungus finds opportunity for adopting a parasitic life. A sharply 
defined boundary line between the innermost elements of the gelatinous felted layer 
and the outermost of the later rind cannot be drawn in the earlier stages of 
development. 
The assumption by the tissue of its ultimate form begins with the thickening 
and turning brown of the walls of the hyphae. It advances on the transverse 
section from without inwards, and its first beginnings may be followed upwards 
to the base of the young apical cone. As the coloration advances the gelatinous 
felt which covers the rind dries up and usually no trace of it remains in older 
strands. At the same time the formation of the ultimate medullary hyphae begins 
inside; these arise, as is shown in Fig. 12, as slender lateral branches from the 
cells of the zone which produces the medulla, and from the innermost layers of 
the rind which are not sharply distinguished from it; these branches elongate 
and ramify and enter the axile cavity, and becoming woven together there fill it 
up in the manner which has been already described. As the zone which produces 
the medulla always consists of several layers of cells, the hyphae which proceed 
from its outer layers into the cavity must force their way between the inner layers, 
which may become much displaced and squeezed together, and this gives rise to 
the irregularly constructed boundary zone between the medulla and the rind which 
was mentioned above. A subterranean strand may form branches of the same 
kind in varying number and with no regular arrangement. At the point where 
a branch subsequently appears, a new formation in the form of a thick cushion 
of pseudo-parenchyma is developed within the inner layers of the rind by shoots 
from its cells, and the growing point of the strand emerges in a few days from 
