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Scientific Memoirs by
measures about 33 x 30µ. Such spores never occur independently. They are
always associated with normal uredospores, and as they appear to rise from the
same sporogenic tissue as the latter do, they must provisionally be regarded
as a second form of teleutospore of R. sessilis.
The series of events occurring in the annual cycle of development of
R. stictica is essentially parallel to that present in the case of R. sessilis, but
differs in regard to certain details, which are partly intrinsic and partly dependent
on differences in the characters of the host plant. As in the case of the other
species the cycle may be regarded as commencing with the appearance of the
new crop of leaves on the host subsequent to the complete defoliation which
takes place at the beginning of the hot weather. The leaves which fall at this
time are universally coated beneath by a dirty brownish stratum, consisting of
innumerable teleutospores and a sprinkling of uredospores. The trees are bare
of leaves for a few days only, and then become covered by a crop of brilliantly
green new ones, and these very soon show signs of infection in the appearance
of minute, dull ochreous spots, consisting of groups of spermogonia (Fig. 8,
P l a t e I I ) , O n t h e u p p e r s u r f a c e s o f t h e l e a v e s t h e s p o t s a r e s l i g h t l y e l e v a t e d , a n d
on the under ones correspondingly depressed. Each patch, as in the case of
the corresponding fructification in R. sessilis, passes right through the leaf, the
tissues of the host being interwoven with a dense web of mycelial filaments con
-nected on either side with cushions of sub-cuticular, small-celled elements. The
groups vary in size greatly, but large ones may measure as much as .625 mm.
in diameter. When the spermogonia first appear they show as minute hard
whitish prominences on a surface of apparently unaltered host tissue. After
- w a r d s t h e y g r a d u a l l y b e c o m e b r o w n i s h p e r i p h e r a l l y , w i t h a c e n t r a l o c h r e - c o l o u r e d
area, where they ultimately open. Finally, pale yellowish masses of spermatia
escape from the orifices, apparently involved in a mucoid basis (Fig. 10, Plate
II). By this time the spermogonial areas of the lamina have become somewhat
deeper green than the rest of the surface, and are somewhat elevated or depress
-ed according to the side of the leaf on which they are situated. A narrow
yellowish peripheral zone is sometimes recognisable around the patches. Still
later, and when uredosporic pustules are beginning to break out peripherally, the
spermogonial areas become brownish. The spermogonia on the upper surfaces
of the leaves are much more conspicuous than those on the lower ones, due to the
relatively small development of uredosporic pustules which takes place around
them.
The deeper tissues of the laminæ within the spermogonial areas are every
-where interwoven by delicate, septate mycelial filaments with intracellular haus
- t o r i a , w h i c h a t t h e l e v e l o f t h e e p i d e r m i s b e c o m e c o n t i n u o u s w i t h m a s s e s o f c e l l s ,
passing upwards around the cells of the host and uniting to form dense cushions
of small-celled tissue constituting the bases of the spermogonia (Figs. 9, 10, Plate
II). From these, dense masses of erect, more or less convergent filaments arise,