Notes . 
445 
of growth of the fungus under these abnormal conditions, and of discovering to what 
extent the hyphae penetrated into the intercellular spaces of the internal tissues, and 
whether haustoria (normal or otherwise) were produced by these hyphae. 
A rapid survey is first made of our present knowledge of the mycelial 
characteristics of the Erysiphaceae in relation to their parasitic habit. The species 
of the Erysiphaceae were regarded since De Bary’s time as strict ectoparasites, until 
in 1899 Palla discovered the semi-endophytic habit of the genus Phyllactinia. With 
this exception the species of the Erysiphaceae , so far as they have been investigated, 
have been found to be strictly ectoparasitic in habit, the hyphae of the mycelium being 
confined to the external surface of the epidermal cells (never gaining access to the 
intercellular spaces of the internal tissues), and merely sending haustoria either into 
the epidermal cells alone, or, in the case of one species, into the sub-epidermal cells 
as well. 
The experiments carried out and the methods employed in the present 
investigations are then described. The fungus used was the conidial stage of 
Erysiphe graminis> DC., a strict ectoparasite under normal circumstances. Young 
leaves of oats and barley were cut off from seedling plants, and a minute piece 
of tissue was cut out with a sharp razor from the upper surface of the leaf. In this 
operation the upper epidermis was removed, and often a considerable amount of the 
mesophyll also, so that in inoculation the conidia were sown on the sub-epidermal or 
deeper layers of the exposed mesophyll, or even on the internal surface of the lower 
epidermis. After inoculation, the leaves were placed on damp blotting-paper in 
a Petri dish. By the sixth to eighth day vigorous infection had nearly always resulted, 
the surface of the wound bearing patches of clustered conidiophores. The leaves 
were then fixed in Flemming’s fluid or in chromacetic, and subsequently embedded 
in paraffin, microtomed, and stained with Diamant fuchsin and Lichtgriin. 
It was found on examining such wounded leaves that the fungus had invaded 
the internal tissues to a remarkable extent. Where the mesophyll-cells remaining 
uninjured were several layers deep, the hyphae had penetrated inwards, winding 
through the intercellular spaces as far as the internal surface of the lower epidermis. 
Haustoria were sent into the cells of the superficial layer of the mesophyll by the 
hyphae creeping on the surface of the wound, and into all the deeper layers of the 
mesophyll by the hyphae running in the intercellular spaces. The cells of the lower 
epidermis were also attacked, the internal wall having been penetrated. The sheath- 
cells of the vascular bundles were much invaded by very vigorous haustoria. The 
haustoria formed in the cells of the internal tissues resemble in every way those 
which occur normally in the epidermal cells. 
The hyphae enclosed in intercellular spaces, either just below the surface of the 
wound or deep down in the internal tissues, struggle to produce conidiophores. The 
respiratory cavities over the stomata of the lower epidermis were in a great number 
of cases full of vigorous hyphae producing young conidiophores. When the inter- 
cellular space, where the young conidiophore was produced, was shut off from the 
open air by only a thin membrane consisting of the walls of collapsed mesophyll- 
cells, the young conidiophore growing upwards, sometimes proved able to break 
through it and continue its growth. The direction of growth of the young 
