24
Scientific Memoirs by
all. Around the surface of the epidermal cells, and between them and the
cuticle, every uredosporic pustule shows a dense cushion, or disc, of small-
celled fungal tissue, from which the stalks of the uredospores arise. The bed
from which the uredospores arise is therefore in this species quite superficial,
and, due to this, the stalks often remain adherent to the spores when detached
by friction subsequent to the rupture of the cuticular covering of the pustules.
An account of the structural details of the spores will be found in connection
with the data regarding the subsequent crop of them.
No further development or diffusion of the parasite occurs in the leaves
which are primarily affected, and at the end of the hot season it is represented
simply by a set of groups of dried-up spermogonia, surrounded by varying
numbers of concentrically arranged uredosporic pustules. With the onset of
the rains a fall of leaf, almost, if not quite, as complete as that in the beginning
of the hot weather, sets in, and a new crop of much larger leaves makes its
appearance. This soon shows abundant evidence of the presence of the para
- s i t e , n o t , h o w e v e r , a s i n t h e c a s e o f t h e p r e v i o u s c r o p , b y a p r e l i m i n a r y e r u p t i o n
of spermogonia. These are entirely absent and groups of uredosporic pustules
are developed directly (Fig. 14b, Plate I). These, both in their arrangement
and in individual characters, differ somewhat from their predecessors. When
they first appear they do not show any definite tendency to assume a concentric
arrangement (Fig. 14b , Plate I), and to the end they never have it so strongly
marked as those of the previous crop (Fig. 14a, Plate I). The groups are,
moreover, naturally distinguished by the absence of the central groups of black
points representing dried-up spermogonia. The individual pustules of this crop
are considerably larger and more prominent than those of the preceding one;
they are generally more or less circular in place of elongated, and they have a
rich brown in place of a pale umber tint. They occur on both surfaces of the
pinnules, but more abundantly on the lower ones. This is illustrated by the
following figures. The pustules present on seven pinnules of a leaf were
counted and found to number 126, but of this total only 37 were situated on the
upper and 89 on the lower surfaces of the pinnules. They make their appear
-ance at first in small numbers and quite isolatedly, but, as time goes on, the
numbers increase and the pustules form distinct patches.
The mycelium corresponding with the pustules presents the same general
features as that of the pustules of the previous crop, being relatively incon
-spicuous in the central strata of the leaf and forming dense sub-cuticular sporo
- g e n o u s b e d s . T h e s i z e o f t h e s p o r e s , w h e n d r y , a p p e a r s t o b e t h e s a m e f o r b o t h
c r ops , be i ng a bout . c 31 x . 015 mm. , but , whe n we t , i s pos s i bl y a l i t t l e l e s s i n t he
spores of the second crop, in so far as length is concerned, probably due to
slight differences in the nature of the epispore, which is somewhat thinner and
less deeply coloured in them than in the spores of the previous crop. The stem
is about .005 mm in thickness and of various lengths, up at any rate to .06