A NEW GENUS OF PARASITIC ALG&. 313 
after having attained a more or less considerable size. A still larger number, however; 
are utilized by fungal filaments, and in association with these go to the development of 
patches of a heteromerous lichen. The leaves on which this occurs begin, shortly after 
the onset of the dry weather of the winter months, to show conspicuous dry superficial 
patches of a grey colour, and which, on close inspection, are seen to be composed of 
colonies of minute whitish circular disks (Pl. XLIII. fig. 8). In some cases almost the 
entire surface of the leaf is covered; but in general the groups are more or less isolated, 
circular, and mingled with distinct patches of the Alga in various stages of growth. On 
moistening such lichenoid patches the grey colour disappears in great part; and the 
separate disks now appear as circular green spots shining through a semitransparent veil, 
which invests them and unites them to one another. The patches come in course of 
time to be besprinkled with minute elevated black specks, and ultimately show a greater 
or less number of flattened circular apothecia, with black raised margins and brownish 
contents (Pl. XLIII. fig. 9). 
On removing such a patch (a process which is readily accomplished, as it adheres but 
slightly to the surface of the leaf), it is found to be composed of a basis of delicate colour- 
less membrane, passing continuously over the grey disks and the intervening spaces, and 
in many instances readily separable from the disks save towards their centres. On 
examining such a detached flake of membrane and its attached disks, the former is 
found to consist of a dense web of empty interwoven hyphze, whilst the latter are readily 
recognizable as primary disks of the Alga, which have undergone greater or less modi- 
fications (Pl. XLIII. fig. 10). The cells of the disk are in great part empty and colourless ; 
but beneath the colourless framework or skeleton is a great mass of circular cells. These 
are quite distinet from one another ; and many of them become detached from their places 
in the process of preparing the specimen, and may be found floating free in the sur- 
rounding fluid (Pl. XLIII. fig. 18). The larger number of them, however, remain in 
situ, and appear to be embedded in a granular matrix. Entangled in the flake, in some 
cases, other primary disks may be encountered, which are as yet completely unaltered, 
or exhibit various stages of transition to those just described. The black specks in the 
patch are now resolved into spermogonia full of minute spermatia, and the apothecia 
present all the characters of those of gymnocarpous lichens (Pl. XLIII. figs. 9 & 10). 
The following account of the details of structure and the history of development of 
these lichenous patches is founded on very numerous careful examinations of specimens 
. in every stage of formation. Before any of these lichenous patches had appeared, and 
whilst the study of the development of the primary disks of the Alga occupied attention, 
it was frequently observed that fungal filaments had become attached to the latter 
(Pl. XLIII. fig. 12). These filaments were colourless, ramified over the surface of the leaf, 
and, where they came into contact with the disks, were frequently bent at right angles 
to their previous course, becoming closely applied to the margins of the disks, adhering 
firmly to them, and sending numerous branches over their surfaces. On following out 
the subject more closely in connexion with thelichenous patches, the subsequent history 
of these adherent filaments was ascertained to be as follows :—After becoming adherent 
to the disks the hyphz proceed to ramify abundantly, and eventually form the densely 
