HELIOTROPISM OF ASCI IN DISCOMYCETES 301 



In the Discomycetes the asci and usually also the ascospores ^ 

 are colourless, and the coloration exhibited by the hymenium, 

 e.g. scarlet in Sarcoscypha coccinea, S. protracta, and Ciliaria scutellata, 

 orange in Peziza aurantia, brown in Aleuria vesiculosa and Galactinia 

 badia, and yellow in Cheilymenia vinacea, is due to pigments con- 

 tained within the paraphyses. These pigments may be nothing but 

 by-products of metabolism. On the other hand, there is the possi- 

 bility that they may have some significance for the production and 

 liberation of the spores. It is difficult to avoid the supposition that, 

 in species where the fruit-bodies are exposed to the sun, a pigment, 

 by absorbing the sun's rays, must be a factor in raising the tempera- 

 ture of the hymenium and, therefore, indirectly, a factor in hasten- 

 ing ascus-development and spore-discharge. Falck has actually 

 observed that, when quiescent fruit-bodies of Morchella esculenta 

 which contain a brown pigment are exposed to direct sunlight, 

 not only is the temperature of the hymenium raised but spore- 

 discharge is initiated. 2 



An ascus, like most hyphae with a free end, increases in length 

 by apical growth. It therefore seems probable that the part of the 

 ascus which is sensitive to the heliotropic stimulus of light is its 

 apex. It may well be that in every ascus the operculum originates 

 in a radially symmetrical position at the end of the ascus and that 

 the ascus attains heliotropic equilibrium only when the protoplasm 

 underlying the operculum is symmetrically lighted. This theory of 

 heliotropism can be extended to all the Discomycetes with helio- 

 tropic asci. It has the advantage of simplicity and is in accordance 

 with the modern view of the nature of heliotropic reactions held by 

 Blaauw and others. It accounts for the fact that in all Discomycetes 

 with heliotropic asci the opercula (or the equivalent in the in- 

 operculate species) at the end of the heliotropic reaction do actually 

 face the strongest rays of light. 



The advantage to the fruit-body as a whole in the opercula facing 



1 The Ascoboli are exceptional in having ascospores with pigmented walls. The 

 colour of the disc of Ascobolus stercorarius is somewhat greenish owing to the yellow 

 colour of the mucilage (derived from the paraphyses) combining with the violet 

 colour of the spores. The Bulgariae, e.g. the well-known Bulgaria inquinans, have 

 brown spores. 



2 Vide infra, p. 321. 



