— 175 — 



could have taken place from newly formed conidia. Probably 

 these diseased areas arose from a fresh growth of mycelium that 

 had hibernated in the leaf. The mycelium in the leaf does not 

 descend in the winter through the petiole to the stem and roots, to 

 reascend through the new petioles in the spring. At least in the 

 considerable number of sections made in the spring, no trace of it 

 was discovered in the petioles. 



THE REPRODUCTIVE ORGANS. 



The mycelium is densest in growth just beneath the epidermis 

 of the white area or at its borders, and from this region certain 

 branches break through the upper epidermis of the leaf. (Figs. 

 2 and 3.) The ends thus exposed to the air undergo a simple 

 segmentation, the lower cells forming the so-called basidia, the 

 upper being the conidia, the reproductive organs of this stage. 

 As conidia fall new growths and segmentation of the basidia form 

 new conidia, and this continued production may go on throughout 

 the life of the host. The chief development of the hyphae and ba- 

 sidia is from the upper surface rather than through the stomates 

 or breathing pores of the lower epidermis. 



The conidia are oblong or C54indrical, and are .026-. 042 milli- 

 metres long by about .0035 m.m. broad.* If 

 they fall on or are blown to a fresh leaf- 

 surface and lodged in a little dew or moisture, 

 tliey send out in a few hours white germ- 

 tubes (Fig. 5). These bore their way through 

 the epidermis of the leaf, and produce new 

 centers of the disease, i. e., new spots. 



In the latter part of the season the mycelium 

 becomes compact in certain places (Fig. 3). 

 Toward winter masses of the mycelium, either 

 ovoid or columnar in form, made up of poly- 

 gonal cells, the outer brown or black-walled, force 

 their way through the broken epidermis anywhere in the diseased 

 part of the leaf, and may be seen as blackish specks, by the naked 

 eye. These are very numerous on our plants and have been 

 termed by Treleaset '' sclerotia:' If the leaves supporting them 

 are brought, in the winter season, into a warm room, and kept in 

 moist atmosphere, hyplial filaments will develop from the surface 



Fia. 5. 

 nating. 



-Conida germi- 



* One millimeter = -}r, of an inch, nearly. 



t Second Ann. Report of the Wise. Agr. Exper. Sta. (1885) p. 52. 



