34 TIIK AMKKICAN MONTHLY [Frl.ruaiT, 



a small qiiaiUitv of moisture, ami it ib at tliis point that the para- 

 site very tVeciuently Inst shows itself; the sccomlaiy spores heinii; 

 caught, ami their germination favored, hy the film of water 

 present ; nevertheless, it may he stateil that a yellow patch is 

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

 cin be detected, ami which appears to be entirely due to the 

 action of water. 



The disease is frequently proiluceil by direct application of the 

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

 and althouixh morpholoj^ically there appears to be but one species 

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

 termed tlistinct bit)lo«j;ical forms of tliis species ; the secondary 

 spores produced by the fun<:[us developed on one species of host- 

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

 l)elon«;inj:; to a diHerent j^enus, altlioujjjh both species of hosts 

 have their own torm of the fun<^us. 



The sccond;irv spores usuallv j^erminate within twelve hours 

 of beinjj placetl in water, and emit from one end a sinjijle, 

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

 this mycelium, when proiluced 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 acquirinj^n thickness of 

 S-io,'t,antl becoming transversely septate, the cells averaging 2-3 

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

 hyphiE become tinged brown, and with age arc dark brown and 

 nearly opaque. Thinner lateral branches are given ofi' at inter- 

 vals by the hypha:? described above ; these at once pierce the wall 

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

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

 the expense of the contents of the cell. 



Within a fortnight after first entering the leaf, tiie 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 hyphtc 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 120-150/'. in length by 8-10/' 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 base is inflated. During the growth of 

 the sp(jrophc)res, the basal cells of the sclerotioid base send out 

 colorless hyphae ; these penetrate the cells of tlie host for the 

 purpose of obtaining food (Fig. 9). 



At the moment of maturity, the spores germinate within 

 twelve hours when plaqed in a suitable medium. When gcrmi- 



