MYCOLOGICAL SECTION. 119 



turn brown, a central portion sometimes becoming ligbter colored, 

 and, if tlie disease is allowed to progress, they increase in size until 

 the entire leaf becomes browned and dried up. 



The fungus. — The fnngus of the Celery-leaf Blight {Cercosjmr-a 

 Apii, Fries) belongs to a genus numbering over 230 species, all of 

 which attack the living leaves of plants, and many do serious injury 

 in this way to our cultivated crops. 



Our knowledge of these fungi is very imperfect. We only know 

 them in their most active state, v^hen they do the most harm by feed- 

 ing upon the foliage of the plants they infest. We have, however, 

 sufficient reasons for believing that the species now included in the 

 genus Cercospora represent only a state in a metamorphosis that 

 loads to some higher or more perfect, but as yet unknown, form ; in 

 other words, they are comparable to the larva3 of destructive insects 

 that are only known in their larval state. In each of the stages in 

 the development of these fungi we may presume that there are pro- 

 duced special spores or reproductive bodies, which serve to multiply 

 the species, and it is very likely that if we had a complete knowlecigo 

 of these forms we would find the task of remedjdng the evils they 

 inflict much easier than it now is. These remarks are applicable to 

 the species of many other recognized genera which we will have to 

 discuss, and indicate in a brief way the line of investigations that 

 must be made and their importance and magnitude. 



The spores, or, more definitely speaking, the conidia, of the Cer- 

 cospo/'rt in question are from 20 to 80/^* in length, straight or slightly 

 curved, and somewhat club-shaped, being about 4At in diameter at 

 their thickest part ; they are colorless, transparent, and are divided 

 into from 3 to 10 cells by cross-walls or septa (See Plate V.) One of 

 these spores falling upon a celery leaf where there is an excess of 

 moisture will in a few hours develop one or more germ-tubes — each 

 cell composing the spore is capable of thus germinating — which soon 

 find their way into the interior of the leaf, probably by direct pene- 

 tration of the cuticle, and there make a considerable growth in the 

 loose tissues near the back or lower surface. This growth, which we 

 term the vegetative growth of the fungus, destroys the contents of 

 the cells surrounding it, turning the latter brown, and thus producing 

 the characteristic external appearance of the disease. After a tinaa 

 the mycelial threads composing the vegetative part of the fungus bo- 

 come particularly abundant just beneath the stomata of the leaf, and 

 through each of these it pushes outward a number of short, irregular 

 threads or h^^phse, upon the tips of which the spores or conidia are 

 borne (See Plate V, Fig. 2.) The hyphfe vary in length from 30 to 

 80 yu and are between 4 and 5/( in diameter ; they are usually provided 

 with one or two septa near the base. 



Remedies. — The conidia will retain their vitality some time, at least, 

 after they have been thoroughly dried, for I have succeeded in mak- 

 ing them germinate in pure v/ater after they had lain in the herba- 

 rium for several months. Whether these spores will retain their vi- 

 tality through the winter, when exposed out of doors, is unknown, 

 but it is not at all likely that they will. They are doubtless designed 

 for the immediate propagation and dissemination of the fungus; 

 therefore any means which may succeed in j^reventing their forma- 

 tion or hinder their germination will check the disease which this 

 develops. We may accomplish this, in part at least, by watching 



*u is the sign for micromilliiaeter, Oue fi equals ,000039 of an incii. 



