104 RESEARCHES ON FUNGI 



successes. The development of fruit-bodies in the Hymenomycetes 

 generally is accompanied by this phenomenon. In the next Section 

 some of the factors which decide which of the many rudiments 

 shall continue their development will be considered, and it will be 

 shown that light is one of the most important of them. 



Light inhibits the Growth of Rudimentary Fruit-bodies.— 

 Large crystallising-dish cultures, covered with glass plates, were 

 exposed to daylight upon a laboratory table. As more and more 

 cultures were watched, it was noticed that the stipes of fully ex- 

 panded fruit-bodies were usually in contact with the bottom of the 

 dishes (Figs. 58 and 59). Further observations showed that, 

 although numerous tiny fruit-body rudiments came into existence 

 all over the free surfaces of the dung-balls, the particular rudiments 

 which escaped abortion and ultimately developed into mature 

 fruit-bodies always had their origin at or near the bases of the balls. 

 Now this position of origin for the rudiments destined to complete 

 their development is, as we shall see later on, a most important 

 factor in securing the mechanical fixation of the full-grown fruit- 

 bodies, which are organs of considerable height and weight. It is 

 therefore of interest to ask the following question : why do those 

 rudiments which happen to arise on the under sides of the dung- 

 balls, and those only, continue their development while all the 

 others become aborted ? By a series of experiments which will 

 now be described, I have convinced myself that the answer is 

 to be found in the action of an external agent, not gravity or 

 moisture, but light : those rudiments which arise in well-lighted 

 positions are inhibited by the light from further growth, while those 

 which arise in darkness or dimly-lighted positions are not so 

 inhibited. 



The critical experimental cultures were four in number and 

 were arranged as follows : 



Culture A. Dung-balls lighted from above and darkened 

 below. 



Culture B. Dung-balls lighted from below and darkened 

 above. 



Culture C. Dung-balls kept in total darkness. 



Culture D. Dung-balls lighted all over their surfaces. 



