SPORE-DISCHARGE IN THE EXOBASIDIEAE 193 



From the foregoing observations it appears that the relation of 

 general form to function is practically identical in Calocera and- 

 Clavaria. 



Spore-discharge in the Exobasidieae. The Exobasidieae are 

 parasitic in the tissues of living Flowering Plants, produce no special 

 fruit-bodies, and develop their hymenia on the epidermis of their 

 hosts. They thus correspond to the Exoascaceae. Two genera 

 only have been recognised as belonging to the Exobasidieae, namely, 

 Exobasidium with about twenty species and Microstroma with two. 1 



The Exobasidieae differ from the other groups of Hymeno- 

 mycetes in the non-production of a fruit-body. The function of 

 a fruit-body from the mechanical point of view is twofold : to 

 produce an extended surface upon which the hymenium can be 

 developed and to place that surface in such a position that the 

 spores, as soon as they are shot from their sterigmata, may be 

 carried off by the wind. These ends in the Agaricineae, the 

 Polyporeae, etc., are secured only by the construction of a 

 considerable amount of fruit-body substance. Now, in the 

 Exobasidieae, the production of a fruit-body would be superfluous, 

 owing to the fact that the host-plant provides the parasite with a 

 surface upon which the hymenium can be developed and which is 

 favourably situated for the dispersion of the spores. It is very 

 probable that the Exobasidieae had saprophytic ancestors which 

 produced their hymenia on fruit-bodies but that, after the fungi 

 became parasitic, these fruit-bodies underwent gradual elimination. 



Exobasidium Vaccinii occurs on the living leaves of various 

 species of Vaccinium. I myself have seen it in Yorkshire on the 

 leaves of Vaccinium Myrtillus. It causes hypertrophy in the leaves 

 which . it infects, so that they bulge outwards in a bladder-like 

 manner on their under surfaces. The surface of each of these 

 hypophyllous galls is covered with a flesh-coloured hymenium. 

 The hymenium, being thus placed on the lower side of each infected 

 leaf, looks more or less downwards in the same manner as the 

 hymenium of the Agaricineae, Polyporeae, etc. This position is 

 very favourable for spore-dispersal, for there is a space below it 

 from which the wind may carry away the falling spores. The 

 1 Engler und Prantl, Die, nat. Pflanzenfamilien, Teil I, pp. 103-105. 



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