304 DR. D. D. CUNNINGHAM ON MYCOIDEA PARASITICA, - 
insertion, and frequently expand there into one or two blunt diverging processes, giving 
them a somewhat funnel-shaped aspect on section. They taper off from the base, and 
sometimes divide into two or three diverging branches, ending freely among the loose 
tissue of the interior of the leaf in blunted tips. Their thick walls appear to be 
channelled by a number of minute canals, leading from the cavity of the cell to its 
exterior surface ; and the cell-contents consist of a colourless fluid full of minute granules. 
Beneath the stratum of prismatic cells is the loose tissue of the centre of the leaf, con- 
taining air-spaces and an abundance of large spheraphides of oxalate of lime; and this 
is succeeded by one or two layers of tabular or cubical cells, and finally by a thin layer 
of flattened epidermal cells. There are no stomata on the upper surface of the leaf; but 
they occur abundantly below, ranging from 1000 to 1600 per square inch. 
On examining a vertical section of a leaf, including young pustules of the parasite, 
appearances similar to those shown in the drawing are observed (Pl. XLII. fig. 8). The 
filaments of the disk are seen to lie between the epidermis and the subepidermal layer 
of cells. According to the degree of development of the patch, the epidermis over it is 
more or less elevated and separated from the tissue below. In a very young patch, such 
as that to the left hand of the figure, the displacement is comparatively slight; but 
where an abundant growth of ascending filaments has taken origin from the disk, it 
becomes very considerable, and may amount to as much as 0:125 millim. or even more. 
Simultaneous with and proportionate to the displacement of the epidermis, changes 
begin to occur in the subjacent cells of the leaf. A certain amount of sclerosis of their 
walls appears to take place ; but the most conspicuous and considerable alteration occurs 
in the nature of their contents, which become gradually thickened and discoloured, 
passing through various shades of yellow and brown, and ultimately assuming a dry 
granular consistence and bright burnt-sienna colour. These changes at first only affect 
the layer of cells immediately beneath the Alga; but as time goes on they gradually 
advance deeper and deeper into the substance of the leaf, until they extend throughout 
its entire thickness, the affected area corresponding closely with the algal disk. As the 
disease advances the filaments in the centre of the disk become emptied of their contents 
and die; they then dry up, together with the portion of leaf-substance beneath them ; 
and the dry and withered slough, readily breaking off, leaves a hiatus in the leaf, which, 
according to its situation, appears either in the form of a hole punched through the 
surface, or as an irregular space eaten out of the margin. Whilst this destruction of the 
centre of the patch is taking place, the parasite continues to extend peripherally ; so that 
in many instances a great portion of the leaf is ultimately destroyed. The filaments of 
the algal disk appear, as a rule, to be confined between the epidermis and the sub- 
epidermal layer; occasionally, however, branches are given off which force their way 
downwards between the cells of the latter and reach the prismatic layer beneath (Pl. XLII. 
fig. 8). 
The disk is originally composed of a single layer of dichotomous filaments, which 
force their way outwards, between the epidermis and the subepidermal cells. They 
are thick-walled, and at this time (so long, at all events, as they are in active growth) are 
filled with a bright green protoplasm. Whilst the disk continues: to increase at the 
