GERMINATION AND GROWTH. 157 



Hum of P. infestans, which never fructifies there as long as the 

 skin of the tuber is intact. But when, in cutting the tuber, the 

 parenchyma occupied by the mycelium is exposed to the contact 

 of the air, it covers itself with conidia-bearing branches at the 

 end of from twenty -four to forty-eight hours. Analogous results 

 are obtained with the stalks of the potato. It is evident that 

 in these experiments nothing is changed except the contact of 

 the air ; the specific conditions particularly remain the same. 

 It appears, therefore, that it is this contact alone which deter- 

 mines generally the production of the conidiiferous branches.* 

 • The mode of germination and development in the Mncors has 

 been studied by several observers, but most recently by Van 

 Tieghem and Le Monnier.f In one of the common forms, the 

 Mucor pliycomyces of some authors, and the Phycomyces nitens 

 of others, the process is given in detail. In this species germi- 

 nation will not take place in ordinary water, but it readily takes 

 place in orange juice and other media. The spore loses colour, 

 swells, and absorbs fluid around it until double its original size 

 and ovoid. Then a thick thread is emitted from one or both 

 extremities, which elongates and becomes branched in a pinnate 

 manner. Sometimes the exospore is ruptured and detached 

 loosely from the germinating spore. After about forty-eight 

 hours from the first sowing, the mycelium will send branches 

 into the air, which again become abundantly branched; other 

 short submerged branches will also remain simple, or have tuft- 

 like ramifications, each terminating in a point, so as to bristle 

 with spiny hairs. In two or three days abruptly swollen 

 branches, of a club shape, will make their appearance on the 

 threads both in the air and in the fluid. Sometimes these 

 branches are prolonged into an equal number of sporangia- 

 bearing threads, but most frequently they divide first at their 

 swollen summits into numerous branches, of which usually one, 



* De Bary, "Champignons parasitiqnea," in " Annales des Soi. Nat." (4 me 

 ser.), xx, p. 6 ; Cooke, "Microscopic Fungi," cap. xi. p. 138 ; " Popular Science 

 Review," iii. 193 (1864). 



+ Van Tieghem and Le Monnier, "Researches on Mucorini,"in "Ann. des 

 Sci. Nat." (1873), xvii. p. 261 ; Summary in "Quart. Journ. Micro. Science" 

 (2nd ser.), xiv. p. 40. 



