EXTERNAL CONDITIONS AND SPORE-DISCHARGE 121 



not be necessary for the production of the hymenium, when once 

 the hymenium has begun its development, the basidia discharge 

 their spores quite independently of light conditions. 



In the case of certain Ascomycetes, e.g. Ascobolus, it has 

 long been observed that spore-discharge is periodic, a certain 

 number of asci ejecting their spores each day. This phenomenon 

 has come more particularly under my notice in the case of 

 Ascobolus immersus growing on horse dung. The periodicity 

 depends on the alternation of day and night, and can be given 

 an easy ecological explanation. It is important for the purpose 

 of distribution that the ejected spores should be thrown clear of 

 obstacles, e.g. dung balls, &c. The asci are positively heliotropic, 

 and during the day always turn themselves in response to the 

 stimulus of light, so that they point toward open spaces. Such 

 an adjustment would be impossible at night. The periodic 

 discharge of asci each day is therefore of advantage in that it 

 ensures that these structures shall burst only after the light has 

 caused them to take up the most favourable positions for spore- 

 dissemination. On the other hand, as my observations have made 

 clear, spore-discharge in the Hymenomycetes is continuous and 

 does not take place at intermittent periods. The general arrange- 

 ment of a Mushroom or Polyporus is such that, under normal 

 conditions, the basidia never face obstacles. All that is required 

 for the successful liberation of the spores is that the basidia shall 

 shoot them straight outwards from the hymenium for a minute 

 distance. After being violently expelled from their sterigmata, 

 the spores drop into the spaces between the gills, in the hymenial 

 tubes, &c, and thus fall from the fruit-body and are carried off 

 by air-currents without coming into contact with any impediment. 1 

 Almost without exception in the Hymenomycetes, the orientation 

 of the hymenium, and with it the direction of spore-discharge, 

 is finally determined by the stimulus of gravity, and is of such 

 a nature as to ensure that the spores shall fall out of the fruit- 

 body. The perfect indifference to light as regards spore-discharge 

 by the fruit-bodies of Hymenomycetes in comparison with certain 

 Ascomycetes is thus elucidated. 



1 Vide infra, Chap. XVII. 



