INTRODUCTION TO CRYPTOGAMIC BOTANY. 279 



means satisfactory, as is always the case, in perfectly natural 

 groups.* The more imperfect forms are those in which no dis- 

 tinct perithecia are formed, with darker heterogeneous walls, 

 the cells of the stroma at once giving place to the hymenium. 

 Such is the character of the genus Dothidea, but unhappily it 

 is not always easy to decide whether a perithecium exists or 

 not. The multitudinous forms assumed by those genera in 

 which a perithecium does exist, depend upon the degree of 

 development of the mycelium itself, of the stroma which 

 springs from it, its fleshy gelatinous or carbonaceous texture, 

 and of the contained asci. The simpler species consist almost 

 entirely of perithecium, with a very imperfectly developed my- 

 celium. The latter may, indeed, almost always be traced by the 

 microscope on dissection ; but it is often quite invisible even to a 

 good pocket lens. Sometimes they are naked, varying greatly 

 in colour, form, and the nature of the fruit ; sometimes they 

 occupy the tender external tissues of decaying twigs and 

 branches, while occasionally they nestle within the hardest 

 wood. In this case the perithecia are sometimes greatly 

 elongated above, like the neck of a bottle, as indeed they are 

 frequently, when immersed in the stroma, or occasionally when 

 quite free. The length of this neck is not, however, always 

 characteristic in the free species, as it depends in some measure 

 on the degree of moisture to which the perithecia have been 

 exposed ; and M. Duby states that he can at pleasure produce 

 this elongation in species which are ordinarily short-necked. 

 If the mycelium is much developed we have the byssoid 

 Sphcerice. The elongation or distortion of the orifice by 

 which the sporidia are discharged affords excellent sectional 

 characters. Sometimes the perithecia are aggregate and 

 crowded, on a distinct coloured sporiferous stroma, constituting 

 the genus Tubercularia. 



285. The stroma may be developed in various degrees, being 

 more or less intimately incorporated with the bark, or it may be 

 quite free, assuming various degrees of consistence, and, accord- 

 ing to its mode of expansion or elongation, affording very 



* Xylaria pedunculMa, Sow., t. 437, is sometimes reduced to a single 

 sessile perithecium, when it is Spharia stercorarea, t. 357. 



