196 



HARDWICKE'S SCIENCE-GOSSIP. 



ON THE ALTERNATION OF GENERA- 

 TIONS (HETERCECISM) AMONGST THE 

 UREDINES. 



SOME of the readers of Science-Gossip may 

 remember that last year a number of experi- 

 ments were conducted by the writer, upon the alter- 

 nation of generation said to exist between the 

 yEcidium on barberry, and the wheat mildew 

 (Ptucinia graniitiis). The general result of these 

 experiments was this, that of the total number 

 of wheat plants upon which the spores of the 



the result of producing in every instance Mcidiitm her- 

 beridis. The cecidium spores were likewise sown 

 upon young wheat plants, that had never been 

 exposed to accidental infection from the atmosphere, 

 with the result of producing the uredo ; while control 

 plants grown under the same conditions remained 

 free from it. 



The matter, however, does not rest here. There 

 are many uredines said to be heteroecismal in their 

 habit ; if we accept the case of ^cidium berbcridis, 

 and Puccinia graminis, we can hardly refuse to 

 believe in the hetercecism of the other species. This 



Fig. 135. — a, Mcidium berhertdis section ; b c, young spores of tlie uredo o{ Pticcinia graminis ; d c, mature spore of the uredo of 

 Puccinia graminis (cimera) ; f, spores oi JEcidium berberidis (camera) ; g h, teltutospores of Puccinia graminis (camera) ; 

 ij, teleutospores of Puccinia magnusiana (camera) ; k I, teleutospores of Puccinia arundinacea (camera) ; m, teleutospore of 

 Pticcinia magnusiana germinating, each segment of the spore has thrown out a germ-tube, the promycelium of De Bary, 

 bearing promycelium spores (April 17, 1882 ; 48 hours) ; tt, spore oi ^^ciditim berberidis germinating. 



barberry fungus were sown, 76 per cent, became 

 affected with the uredo, which is the first state 

 of the wheat mildew. But upon the other hand 

 no less tliau 70 per cent, of precisely similar 

 wheat plants, grown under the same conditions, 

 but not intentionally infected, became in the course 

 of the same time likewise affected with the uredo. 

 Obviously the slight difference of six per cent, could 

 not be considered conclusive evidence. In the 

 spring of this year a further series of experiments 

 were commenced, in which the wheat mildew of last 

 autumn was sown upon young barberry plants, with 



is accepted generally on the Continent without ques- 

 tion, but in England and America botanists have 

 not, as a rule, regarded these statements with favour. 

 My friend, Trofessor Farlow, has tried a few experi- 

 ments with the American Podisomse, but, as far as I 

 know, no one in this country has taken the trouble 

 to put the matter to the test of experiment. For my 

 own part, it may be said that, having conducted 

 upwards of a hundred cultures during the past two 

 years, I have no doubt whatever upon the subject. 

 If hetercecism be shown to exist with some half- 

 dozen species, it is all we for our present purpose 



