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Scientific Memoirs by
When the host bears young leaves the sporidia attack them, as well as
the green petioles and stems, producing slightly paled areas, on which groups
of very pale spermogonia are developed, very inconspicuous to the naked eye.
Soon after this, primary uredopustules are produced, immediately outside the
group of spermogonia. On the leaf-blade these primary pustules generally assume
a markedly circinate arrangement. Both the spermogonia and the primary uredo-
pustules are produced on both surfaces of the leaf blade, most numerously how-
ever on the lower surface. The pustules on either surface of the leaf-blade are
often exactly opposite one another. On leaf-blades immense numbers of such
pustules are usually formed. Each separate area of invasion is usually about
3 m.m. in diameter; but such areas frequently coalesce. The uredospores con-
tained in these pustules germinate, as usual immediately, and distribute the
fungus largely. A straight germ tube, about 6 µ in diameter, is usually formed,
into the end of which the colourless granular contents wander (Fig. 1, Plate I).
These uredospores produce vast numbers of secondary uredopustules,
without, however, any foregoing spermogonia. These secondary pustules do not,
as a rule, assume the circinate arrangement so common in the case of the primary
pustules. They are, indeed, usually isolated scattered minute brown pustules,
also on both surfaces of the leaf, and also mainly on the lower surface. These
uredospores are morphologically identical with the primary uredospores.
As the vegetative period of the host approaches an end, immense numbers
of teleutospore pustules are formed, thus completing the whole cycle of develop-
ment. The same mycelium which produced secondary uredospores produces
these teleutospores, but mostly apparently in distinct pustules. At any rate,
from the fact that the teleutospore pustules are usually borne on the upper leaf
surface, whilst the uredopustules are mainly borne on the lower surface, the great
majority of the former must be distinct in origin from the latter. I have, how-
ever, found a few uredospores in teleutospore pustules, and some teleutospores
in secondary uredopustules.
There is no vestige of æcidial fructification.
With regard to the experimental evidence that the above life-history is com-
plete, I will give the details of some experiments I made with this view. On
the 24th April, I placed some sporidia, obtained by floating scraped off teleuto-
spores on water in watch-glass (kept in a moist atmosphere to prevent evaporation)
on the leaves of a twig cut off under water, and kept in my laboratory under a
glass shade. On the 9th May (15 days later) I noticed a large patch of paled
tissue on one leaf, with several spermogonia; but on the 21st May many leaves
bore spermogonial patches on numerous paled areas. Again, on the 25th June,
two plants were similarly inoculated with sporidia: they were then placed in a
closed glass gardener's box outside; one (a) was again covered there by a glass
shade, and the other (b) was left uncovered. On the 4th July, 9 days later, I
observed very numerous points of attack on many leaves of each plant. On the