Order 18. =Xylariaceæ.= This order is the most highly developed of the
Sphæriales. The _stroma_ arises on the _surface of the substratum_,
which is generally dead or decorticated wood; it is well-developed,
crustaceous, hemispherical or upright. In the younger conditions
it is covered with a layer of conidia, and later on it bears the
_perithecia_, arranged in a layer immediately _beneath its surface_.
The ascospores are of a dark colour. Often also there are free
conidiophores.
[Illustration: FIG. 122.--_Xylaria hypoxylon_ (nat. size) on a
tree stump: _a_ younger, _b_ an older stroma, both of which, with
the exception of the black lower portion, are covered with white
conidia; _n_, spot where the perithecia are developed; _c_ an
old stroma with upper part fallen off; _d_, _e_ large branched
stromata; _k_ conidia.]
_Hypoxylon_ and _Ustulina_ have a cushion-like
or crustaceous stroma.--_Xylaria_ has a club-shaped
or branched stroma, often several centimetres high. _X.
hypoxylon_ (Fig. 122) and _X. polymorpha_ occur on old
tree stumps.--_Poronia_ grows on old horse dung, and has a
conical stroma.