5 6 Heteroecism. 



proceeded from the formation of teleutospores being succeeded by aecidio- 

 spores, presumably at first both arising from the same mycelium. Next a 

 division of labour took place, and the, mycelium of the aecidia was pro- 

 duced by the sporidiola, while the mycelium of the teleutospore proceeded 

 from the aecidiospore. The advance to heteroecism took place when the 

 aecidiospores produced their mycelium in one host-plant and the : 

 spores, through the sporidiola, in another, and the kerne of the matter lies 

 in the answer to the question, How did this come about? It may either 

 have taken place bv a long series of slow and gradual changes, whereby 

 the different spore-forms gradually accustomed themselves to the new mode 

 of life or it may have developed suddenly by one of the spore-forms 

 terminating .and growing on a different host-plant, and continuing to do 

 so But this latter view is hardly borne out by some experiments conducted 



liss Gibson 1 , in which the aecidia from different host-plants were use 

 to infect Ranunculus ficaria, and while the germ-tube as a rule entered 

 the stoma freely, it was generally dead and shrivelled by the third day. 

 This result was not supposed to be due to starvation, for she says: 

 " Whether the incapacity to penetrate the cells is due to lack ot attractive 

 substance or to the presence of anything actively repellent is not clear 

 though, as before stated, certain facts seem to suggest the presence ot 

 something harmful to the hyphae." 



A few concrete examples may be given to show how far these views are 

 borne out by facts. 



De Bary considered the probable origin of three species of Chrysomyxa 

 occurring in the Alps, and the relation existing between them : C. rlw- 

 dodendri (DC.) De Bary, forms its uredo and teleutospores on species 

 of Rhododendron, while its aecidiospores occur on Picea excelsa, the name 

 given ito this form before its connexion was discovered being Aecidium 

 abietimim, Alb. and Schw. C, ledi (Alb. and Schw.) De Bary, forms uredo 

 and teleutospores on Ledum palustre, and its aecidia also on Picea 

 excelsa, there being little or no distinction between them and those of 

 C. rhododendri. The third, C. ab'ietis (Wallr.) Ung., forms the same kind 

 of teleutospore on Picea excelsa, but the sporidiola from the germ-tube 

 produce mycelia which only form teleutospores and no aecidia or uredo- 

 spores have been observed. In seeking to account for this, he assumes 

 a common origin of the three forms, and considers that either the original 

 form from which they were all derived had no aecidial fructification to 

 start with, or there was an aecidial fructification, and C. abietis has in 

 course of time dropped it. The latter view is the one he favours. We 

 can imagine these three forms competing for Picea as an aecidial host, 

 and while two succeeded in establishing themselves, the third, C. abietis. 

 was compelled to drop it altogether. 



Barclay 2 , in tracing the developmental history of Uredineae, 

 attempted to show that in the struggle for existence, heteroecism was 

 beneficial, and that if two species compete against one another for a host, 

 that which makes for heteroecism will more probably succeed than that 

 which makes for autoecism. 



There is another interesting series of forms worthy of consideration 

 known as " coronate " rusts, because the apex of the teleutospore is pro- 

 longed into and crowned by a number of finger-like processes. There are 

 both heteroecious and autoecious species as follows: 



1. P. roronata, Corda, I. Franguli alnus, II., III., Grasses. 



2. P. lolii, Niels., I. Rhamnus cathartica, &c., II., III., Grasses. 



3. P. himalayensis (Barcl.), Diet. I. R. dahurica, II., III., Grasses. 





