GENERAL INTRODUCTION 7 



vigorous, but when it commences to ripen, tliat is, to die gradually, 

 the same mycelium that had previously produced summer-spores 

 now commences to produce winter-spores, whose general form 

 and function, or use, are totally different to those of summer-spores. 

 The transition is not abrupt wlien the plant first begins to ripen, 

 the clusters of spores often contain both summer and winter forms, 

 and as the ripening or dying of the wheat plant progresses, summer- 

 spores entirely disappear, and are replaced by \\inter-spores. This 

 change in the habit of the fungus we attribute to changes taking 

 place in the host-plant. During active growth the chemical com- 

 position and physical conditions, as to density of the tissues, etc., 

 are fairly constant, and favour the development of summer-spores, 

 but when the ripening or dying stage is reached, the difference in 

 the chemical condition of the sap, also its quantity, and the greater 

 rigidity of the tissues, together prevent the further production of 

 summer-spores, but favour the production of winter-spores. The 

 general significance of this idea will be more e\ident as the student 

 progresses in the stud}' of fungi ; but from the first the fact cannot 

 be overlooked that the great majority of fungi that grow on a host 

 that changes as indicated above, produce two forms of fruit, 

 conidial and ascigerous or winter-fruit, whereas fungi that grow on 

 a comparatively unchangeable substance, as rotten wood, humus, 

 etc., show no change of form, and as a rule produce only one kind 

 of spore, as in toadstools, bracket fungi, in fact throughout the 

 thousands of species included in the Basidiomycetes. It is amongst 

 the rusts generally, and more especially the numerous members 

 of one particular genus, called Puccinia, that we meet with the 

 greatest variety in the number of different kinds of spores produced, 

 and also in growing on different hosts during different periods of 

 their development. Evidence points to the conclusion that all these 

 fungi, at one time or other, possessed the three forms of spore as 

 described under \\heat rust, and also that there was at one time 

 a sexually produced condition. The last has now entirely dis- 

 appeared, functionally, but vestiges remain. At the present day 

 in many species, for unknown reasons, various spore-forms have 

 dropped out, or are omitted in the cycle of development. In some 

 species only the resting-spore or \\inter-spore stage remains. The 

 well-known and destructive hollyhock rust {Puccinia malvacearum), 

 forming small, hard brown warts on the leaves and stem, illustrates 

 this condition of things. This fungus, along with alhes, appears to 

 have solved the problem of effecting their requirements with the 

 least possible expenditure of energy and material. In the majority 

 of instances, a continuous succession of summer-spores through- 

 out the growing season of the host, followed by a crop of winter- 

 spores, are necessary for the continuation of the fungus in space 

 and time. In the hollyhock rust, the only spore formation left 

 corresponds structurally to the winter-spore stage of allied fungi. 



