Heteroecism. 



CHAPTER XVII. 

 HETEROECISM AND ITS ORIGIN. 



In a great many species the various stages of the self-same fungus, 

 as already indicated, occur on the, same host-plant, but the variety in. 

 the mode of reproduction has also brought about a variation in the mode 

 of nutrition, for there are a number of species in which one part of their 

 life is passed upon one species of plant, and the remainder on a totally 

 different species. The host-plants are not even related to each other, but 

 stand far apart in their natural affinities. Those which passed their entire 

 existence on one plant have been called autoecious species, while those 

 which spread it over different plants are called heteroecious species. 



As a general rule it is assumed that the different forms of rust occur- 

 ring on the same host-plant are genetically connected, ialthough it is always 

 desirable, where possible, to have experimental proof of it. In Uromyces 

 fiolygoni, for instance, the three stages of aecidio, uredo, and teleuto- 

 spores may all occur together on the same leaf, or the aecidia may occur 

 on one portion of the plant, and the uredo and teleuto sori on another ; 

 and in both cases the species is regarded as having three stages, which are 

 different forms of the same fungus. It is but a step further to have, say, 

 the aecidia on one host-plant and the uredo and teleuto stages on another, 

 and this affords a greater variety of food supply. Just as in the separa- 

 tion of the sexes in flowering plants, we are justified in assuming that all 

 the different stages occurred at first on' the same host-plant, but gradually, 

 in the struggle for existence, one reproductive body matured on one plant 

 and the others on a different plant, so that a change of food was secuied 

 and a succession of crops insured. 



This heteroecism of the rusts was first discovered by De Bary in 1864, 

 when he proved that the rust in wheat, Pucdnia graminis, produced its 

 uredo and teleutospores on the Gramineae, while its aecidial stage 

 developed on the barberry. 



It has thus been assumed that heteroecious species originated from 

 autoecious species in the simple and seemingly natural way ithat the two 

 generations separated, just to occupy fresh ground, at first passing over 

 to nearly allied plants, and gradually to plants further and further re- 

 moved in the natural system, until the present position of affairs was 

 brought about that the two generations of the same fungus attack plants 

 widely removed from each other, as far as their natural affinities are 

 concerned. 



But there is no evidence to prove that such a gradual separation took 

 place, for even although the species of Pucdnia on Phalaris may have 

 their related aecidia on other Monocotyledons, and the Uromyces on the 

 pea, with its aecidia on a Euphorbia, yet they are always considerably 

 remote from each other. In fact, (the view that heteroecism originated 

 suddenly and without the different generations slowly and gradually pass- 

 ing from plant to plant, seems to have most in its favour. It is admittedly 

 a difficult problem, since from the very nature of the case no one has been 

 able to observe an autoecious fungus becoming heteroecious. 



Heteroecism is only possible when more than one spore-form occurs in 

 the life-cycle, and how the variety of spore-forms originated is capable 

 of different interpretation. It may either have been a progressive develop- 

 ment from the simplest forms, or it may have been a retrogression from 

 the most highly developed forms. We may conceive heteroecism to have 



438. c 



