Notes. 
73 i 
LIFE-HISTORY OF RHYTISMA ACERINUM (PRELIMINARY AC¬ 
COUNT) .—The needle-shaped ascospores of Rhyiisma acerinum are one-celled, 
uninucleate, and furnished with a massive sheath. From experiments carried out in the 
laboratory the spores conveyed by air currents appear to fall exclusively on the upper 
leaf surface. They readily germinate in a weak decoction of prunes, first becoming 
septate into two, rarely three, cells. The germ-tube almost invariably arises in 
a lateral position from the blunted half. The mycelium is exceedingly fine, the 
nuclei being very minute; it develops rapidly within the upper epidermal cells, 
completely filling up the latter. Infected areas first appear as yellow patches which 
very early begin to blacken at the centre ; these areas form stromata which first give 
rise to pycnidia. Increased development of the mycelium in the epidermal cells of 
specified areas in the stroma causes these cells to split in half tangentially, the upper 
half forming a dome-shaped roof and the lower remaining as the floor of the future 
pycnidium. Here the mycelium gives rise to simple conidiophores which abstrict 
small, uninucleate conidia in enormous numbers. The conidia appear at the ostiole 
of the pycnidium in a viscid exudate; their function is not known. After the 
disappearance in late summer of the conidia the persistent conidiophores either 
become concrescent and blackened or, if the pycnidium is to be converted into an 
apothecium, they are raised up by growth of the cells below them and become a part 
of the future apothecial roof. Usually, however, apothecia arise de novo at the margin 
of the stroma, in which case the roof is formed by the addition of cells abstricted from 
the mycelium below. When the process of roof thickening is over there is an 
appreciable space for further development. Among the apothecial cells there now 
appear cells conspicuous by their larger size and contents, which, from their structure 
and orientation, are highly suggestive of archicarps consisting of a basal oogonium 
bearing a trichogyne above as a separate cell; the trichogyne abuts on another 
larger cell, possibly an antheridium. The cytology of these structures is still under 
investigation. No fertilization was observed, but the cells of the archicarp were seen 
in some cases to break down, suggesting a scolecite formation; it is, however, 
probable that these structures are abortive. The paraphyses developed from large 
basal apothecial cells are straight, with pointed tips, and linked together with well- 
marked H-connexions. A transverse section of an apothecium with young 
developing asci show's the latter arising in groups at definite intervals and emanating 
from a complex of cells. At this stage changes are seen to take place within the 
dome of the apothecium in preparation for dehiscence of the latter. A tangential 
slit is developed in the roof, and this gradually widens, forming a cavity filled with 
gelatinous material. The gelatinization is not progressive, but the cavity gradually gets 
bigger, probably owing to absorption of water causing the inner part of the roof 
immediately below the cavity to protrude into the apothecium. This would bring 
about increase of pressure within the closed apothecium, and, owing to the convexity 
of the roof, increase of pressure from below, together with absorption of moisture into 
the cavity, would ultimately bring about rupture of the outer wall of the roof. In 
