34 THE AMERICAN MONTHLY [February, 



a small quantity of moisture, and it is at this point that the para- 

 site very frequently first shows itself; the secondary spores being 

 caught, and their germination favored, by the film of water 

 present ; nevertheless, it may be stated that a yellow patch is 

 often present at the point indicated, in which no trace of mycelium 

 can be detected, and which appears to be entirely due to the 

 action of water. 



The disease is frequently produced by direct application of the 

 secondary spores to damp portions of the surface of the leaves, 

 and although morphologically there appears to be but one species 

 of the fungus under consideration, yet there are what may be 

 termed distinct biological forms of this species ; the secondary 

 spores produced by the fungus developed on one species of host- 

 plant rarely cause the disease when sown on the leaves of a host 

 belonging to a difterent genus, although both species of hosts 

 have their own form of the fungus. 



The secondary spores usually germinate within twelve hours 

 of being placed in water, and emit from one end a single, 

 unbranched, very sparsely septate tube about 1.5-2// in diameter; 

 this mycelium, when produced on the surface of a suitable leaf, 

 soon enters by way of a stoma into the interior. When once 

 within the leaf, the mycelium at first forces its way between the 

 cells of the host, the main branches soon acquiring a thickness of 

 8-io,a, and becoming transversely septate, the cells averaging 2-3 

 times as long as broad (Fig. 9) ; by degrees the walls of the 

 hyphae become tinged brown, and with age ai'e dark brown and 

 nearly opaque. Thinner lateral branches are given oft at inter- 

 vals by the hyphse described above ; these at once pierce. the wall 

 of an adjacent cell, pass into the interior, and tbrm a complicated 

 coil (Fig. 10), their function being that of assimilating food at 

 the expense of the contents of the cell. 



Within a fortnight after first entering the leaf, the mycelium 

 has usually radiated from the point of injection and formed a 

 more or less circular patch about i cm. in diameter, and of a 

 pale yellow color ; this patch has now sunk below the ordinary 

 surface-level of the leaf, owing to the collapse of the epidermal 

 cells. In the meantime numerous short, lateral branches of the 

 primary hyphse have developed a pseudoparenchymatous mass 

 of tissue at the apex, the superficial cells of which give origin to 

 the sporophores (Fig. 9). The sporophores, when fully de- 

 veloped, measure 120-1^0,'/. in length by 8-ro;/ in diameter, and 

 are divided into 6-9 cells by transverse septa. The walls when 

 matured are brown, becoming paler upwards. The basal cell 

 resting on the sclerotioid liase is inflated. During the growth of 

 the sporophores, the basal cells of the sclerotioid ]:)ase send out 

 colorless hyphee ; these penetrate the cells of the host for the 

 purpose of obtaining food (Fig. 9). 



At the moment of maturity, the spores germinate within 

 twelve hours when placed in a suitably medium. When germi- 



