EFFECT OF SUBSTRATUM ON I >KVKI.< 'I'.MENT uF 1'A HAM 1 K. 17 



host, but soon die nil', if it le nut a suitable one. lie l'>aiy 

 also observed u germ-tube of Peronospora /'////'"""' which 

 frequents Anemone, making its way into /A/// ////*/////., F';i,-in, 

 but soon to die. Germinating spores of ('//xfnjuix < ml'nln* will 

 enter the stomata on leaves of any of their host-plants, e>p.-i -ially 

 /',ij,.-<,'//it t but will only develop further if they are su<-ceful 

 in penetrating into the cotyledons. 



Variation in the substratum produces very great difference in 



the fnrmation of the reproductive organs. Thus many I'stila- 



gineae produce conidia by continuous sprouting only when 



cultivated in nutritive solutions, while their resting-spoivs are 



developed only from a mycelium which inhabits the reproductive 



organs of their host; this is the case with Cxfi/tti/o carids, 



U. initli'iiriiin, and (\ ///////'. In others the spores are found in 



all parts of the tiower, and even in the inflorescence, as in 



UstiJ't'j" cruenta and U. //"//. y^/,,/,/x, while in Ustilinjn //<////,//* 



spores are also produced in leaves and stems. 



The various parts of the same plant behave very differently in 

 this respect. The Ustilagineae just considered reproduce them- 

 selves only on certain organs of their host, although the 

 mycelium is also present in other organs. Other fungi behaving 

 similarly are /:'///'</'//</, f //;>// i/m which produces its perithecia only 

 mi the surface of the sheath of one of the leaves just below the 

 inflorescence; Aecidium elatinum develups its aecidia only mi 

 the needles of the witches' broom: Aecidium nji/mrliii" has its 

 .i.-cidia only on the leaves of its host; JK.runxmx /in/ /// has a<ci 

 only on the fruit; 0" ///j>f< >-</>' >/'n produces teleutuspores in the 

 epidermal cells of the stem, never of the leaves ; and so <>n in 

 many other cases. 



The formation of oo-onia of f '//X/H////.S- exhibits a striking vari- 

 ation according to the host-plant. ( '//.s/<yy//s ," ml.iilns. on f'<iji*//n 

 produces conidia alone, never oogonia : yet the latter are plenti- 

 fully developed ill tloWers of BrOSSlCa, Ix'illg coiltilled, houever, to 



the llowers, while conidia are produced in all parts. i y/.s7,y>//s 

 A//// forms conidia only in the leaves, and oogonia only in the 



ma of Aiii'ii-iintlm* ill it a in.^ 



The mycelium of many other fungi can only -row in certain 



OrganS, While genii-tubes tVollI the Spores aiv only able to pene- 

 trate into certain pails of the ho-t. Tin:- /.' >ascus //<i incaiun 

 '!)' lin-y, Moi-ffi'it'ii/i/ n,,,/ I: ' /''///', l-'.n.'liNli K.litii'ii, \<. :i!M. 



