PERISPORIALES 



41 



ample is seen on the leaves of the common lilac which is almost 

 universally covered with one of the species of this order. Some 

 species sometimes appear on young stems and fruit as well as 

 leaves. One species is common on the peculiar knot-like fascicles 

 of twigs that disfigure the hackberry tree of the Mississippi valley 

 region, and another is confined to the brown rust-like masses 

 common on the leaves of the beech. The fungus draws nourish- 

 ment from its host by means of haustoria, which are mere ex- 

 pansions of its hyphae. Two kinds of reproductive bodies are 

 present : (i) Conidia, which are produced by the successive cut- 

 ting off" of the ends of erect hyphae. (/Y. 4. f. 4.) These consti- 

 tute the summer spores, and are blown to other leaves, germinate 

 quickly and rapidly spread the growth of the parasite during the 

 growing season. (2) Perithecia, which are spherical bodies with 

 a thickened protective wall and contain the ascospores enclosed 

 in membranous sacs (asci). {P/. 4. f. 5.) These first appear as 

 minute whitish bodies soon changing to yellow, brown and finally 

 black ; they can be easily seen with a hand lens ; all stages 

 can frequently be seen at the same time on one leaf. It has 

 been supposed that these perithecia are produced as the result of 

 sexuality. {P/.4. f. 6, 7, 8.) In any event the ascocarpis developed 

 after a i)lan similar to the homologous organ in some of the 

 higher algae where sexuality is the exciting cause. The asco- 

 spores remain over winter and germinate the following spring. 



The perithecia are mostly provided with a series of appendages 

 which have different forms and serve in part as the basis of separ- 

 ation of genera. Some of the=e appendages are needle-like and 

 provided with a bulbous enlargement at their base, some are 

 coiled or hooked at the end, others are dichotomously divided 

 often into an elaborate pattern, others still are similar to the 

 ordinary mycelium, tho often differently colored. 



One species produces the common powdery mildew of the 

 grape, which is also very common on the Virginia creeper ; another 

 produces a common disease on young cherry and plum trees ; an- 

 other forms the destructive hop-mildew, a fourth attacks wheat 

 and other grasses, and still another the gooseberry. There are 

 species found on the maple, elm, oak, basswood, hackberry and 

 numerous other trees ; on the lilac, willow, huckleberry, haw and 

 various other shrubs, and on a great variety of herbaceous plants 



