248 DIVISION II.—COURSE OF DEVELOPMENT OF FUNGI. 
The shape of the pycnidia isin general the same as that of perithecia or spermo- 
gonia, and the internal cavity, like that of the spermogonia, is according to the species 
either simple or divided by projections from the wall into usually irregular narrow 
compartments communicating towards the orifice. The pycnospores of the different 
species exhibit the usual variety of modifications in the structure of spores. Two 
extreme forms may if necessary be distinguished : small-spored pycnidia (corresponding 
. to the old form-genus Phoma) with very small, colourless, somewhat elongated spores, 
which are imbedded in mucilage and are discharged in masses from the orifice of the 
pycnidium, as in Pleospora and Cucurbitaria elongata, and Jarge-spored pycnidia 
with comparatively large either simple or compound spores, the walls of which are 
often thick and of a brown colour. 
Section LXXII. In the species of which we are now speaking, as well as in those 
previously described and which have been thoroughly examined, the different forms of 
gonidia and gonidiophores occur according to the species in the greatest 
possible variety of combinations with the perithecia and sometimes with 
one another. Examples of this, such as may be considered to be well ascertained, are 
to be found in the works here quoted, and especially in Tulasne’s Carpologia. Some 
of the accounts contained in this book, and still more those in the more recent de- 
scriptive writings, must be accepted with caution. We may here call attention more 
distinctly to the fact, which may be gathered indeed from some of the previous remarks, 
that each species has the faculty of forming asci and gonidia within broader or 
narrower limits as the result of its inherent inherited qualities; external causes, 
especially the quantity and quality of the food at its disposition, then determine in a 
variety of ways the phenomena which are actually observed. A few examples will 
now be given in illustration especially of the latter point, which will be further 
discussed in section LXXIII. 
The genera Ustulina, Poronia, and Hypoxylon among the Xylarieae may be con- 
sidered, as far as we at present know, to be plants of one form, since, like Epichloe 
(and Claviceps), they produce gonidia of ome definite form on the young stroma, 
and then perithecia. 
Cucurbitaria Laburni! forms in the dead rind of Cytisus Laburnum large flat 
roundish cushion-like stromata which reach a breadth of some millimetres and finally 
issue from the ruptured periderm covered with numerous black round spore-receptacles. 
Some of these are perithecia, some gonidial receptacles, pycnidia, with a single cavity 
and a narrow canal at the orifice ; a stroma according to Tulasne may bear only peri- 
thecia or only pycnidia, but usually has both and more than one kind of pycnidium. In 
the latter case the receptacles make their appearance on the stroma, which increases 
in size while they are forming, in adox? the following order in time and centrifugal 
succession :— 
1. In the middle of the stroma one or a few comparatively large colourless pycnidia, 
producing colourless thin-walled unsegmented cylindrico-elliptical spores 5-10 u in 
length, on short sterigmata. 
2. Numerous pycnidia with thick black walls in which are abscised from short 
sterigmata 
(2) Colourless spores of very unequal size, 

1 Tulasne, Carpol. II, p. 215, t. XX VII. 
