28
Scientific Memoirs by
advances the stem-cells and the basal cells dry and shrivel up, save the cystic
protrusions of the latter, and ultimately the head is detached from the stem,
usually due to the excessive upward pressure of younger spores. The stem cells
are now once more set free from one another and remain as a series of long
hair-like processes on the surface of the pustule (Figs. 17, 18, Plate I). The
length of these hairs is very considerable, in some cases being as much as
.066 mm., and renders the specific name of the species very inappropriate.
The epidermal cells of the host beneath the pustules are specially rich in pro-
toplasmic content, and are also of somewhat smaller size than the corresponding
cells elsewhere, either due to pressure or to processes of proliferation similar to
those often occurring in the case of the algal elements of a lichen. The sub-
epidermal cells beneath the pustular areas are also characterised by excess of
protoplasmic content, although not to such a high degree as the epidermal ones.
The mature detached teleutospores, when first separated from their stalks,
consist of flattened concavo-convex, more or less circular masses of cells. The
convex surface is covered by a thick, deep brown epispore, and surrounded by a
row of short tubercles (Figs. 5, 6, Plate I). The under-surface presents a
marginal, elevated, somewhat convex rim, corresponding with the true outer, but
now under, surfaces of the marginal row of spore-cells. Within this is situated
the row of cystic protrusions of the outer portions of the peripheral basal cells,
which in the normal fresh spores pass more or less vertically downwards as a
fringe of short ,colourless, highly refractive blind tubes, but which in dried speci-
mens very frequently become flattened out, so as to form a horizontal frill around
the spore (Figs. 1, 2, 3, Plate I). The central deeply-concave area of the
under-surface corresponds with the shrivelled central basal cells and central
portions of the peripheral ones (Fig. 7, Plate I).
The cystic protrusions of the peripheral basal cells do not persist beyond a
certain period in the mature spores. Their function is apparently to facilitate
the adhesion of the spores to surfaces with which they may come in contact,
and with the rest of the basal cells they soon disintegrate and disappear. The
spores, therefore, at different periods present very different appearances. Newly
matured ones show the cysts very clearly, while in many old ones not a trace of
them remains. Intermediate forms occur, showing the cysts in various stages of
disintegration. The cysts at an early stage are, as previously mentioned,
strongly refractive and appear plump and full. Subsequently, they become
much dilated, and at the same time their contents shrivel up into threads and
their walls become very thin, save along their lateral surfaces. Ultimately the
thin portions of the walls disappear, and the thicker ones persist as a series of
fine filaments fringing the spores and looking not unlike fine mycelial threads.
Other thread-like elements also are present in some cases, due apparently to
persistent remnants of the cell contents remaining after the solution of the walls.
When the cysts and basal cells have disappeared, the spores, when viewed from