OARPOPHTTA. 165 



Possibly they exist in the dense tissues of the knot, and 

 fertilization probably occurs in the spring or early summer, 

 while the conidia are being produced on the surface of the 

 young knot. 



345. The parasitic filaments of each year's knot gener- 

 ally penetrate downward some centimetres into the unin- 

 jured bark, and remain dormant there until the following 

 spring, when they begin the growth which results in the 

 production of a new knot, as described above. 



346. The Black Fungi include a large number of exceed- 

 ingly injurious species; they often attack and destroy not 

 only plants, but also insects, upon which their ravages are 

 in many cases very great. 



347. To this order belongs the Ergot (a common para- 

 site upon heads of rye), and also many of the black growths 

 upon the bark and wood of trees. Many species produce 

 black spots upon living leaves, while many others occur 

 upon dead leaves and twigs. 



Practical Studies. — (as) In early summer examine the Choke-cherry 

 and Plum trees (wild and cultivated) for the young stages of Black 

 Knot. Watch the development until the knot becomes velvety in ap- 

 pearance (about midsummer). Now make very thin cross-sections 

 of the knot and examine for conidia. 



(J) Late in autumn and in early winter examine the knots on the 

 same trees. Note the young perithecia, i e. hollow papillae. Make 

 very thin vertical sections through some of these. No perfect spores 

 can be found at this time. 



(c) Collect fresh knots in midwinter and make similar examina- 

 tions, when the sacs and spores will be found. 



Note.— The several stages may be readily preserved in alcohol for future 

 study. 



348. The Lichens ( Order Idchenes) are among the most 

 interesting plants of the vegetable kingdom. They are not 

 only often of exceeding beauty, but their structure and 

 their mode of life are in some respects very wonderful. 



