248 DIVISION II. — COURSE OF DEVELOPMENT OF FUNGI. 



The shape of the pycnidia is in general the same as that of perithecia or spcrmo- 

 gonia, and the internal eavily, 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 Fhoma) with very small, colourless, somewhat elongated spores, 

 which are imbedded in mucilage and are discharged in masses from the orifice of the 

 pyenidium, as in Pleospora and Cucurbitaria elongata, and large-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 TJstulina, 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 one definite form on the young stroma, 

 and then perithecia. 



Cucurbitaria Laburni 1 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 Tulasnc may bear only peri- 

 thecia or only pycnidia, but usually has both and more than one kind of pyenidium. In 

 the latter case the receptacles make their appearance on the stroma, which increases 

 in size while they are forming, in about the following order in time and centrifugal 

 succession : — 



1. In the middle of the stroma one or a few comparatively huge colourless pycnidia, 

 producing colourless thin-walled unsegmented cylindrico-eUiptical spores 5-10 /1 in 

 length, on short sterigmata. 



2. Numerous pycnidia with thick black walls in which are abscised from short 



sterigmata 

 / Colourless spores of very unequal size, 



1 Tulasne, Carpol. II, p. 215, t. XXVII. 



