HELIOTROPISM OF ASCI IN DISCOMYCETES 315 



and in good condition. With a hand-razor he cut transverse sections 

 through the alveoh, mounted them in water, and studied them under 

 the microscope ; and he convinced himself that, in each alveolus, 

 the asci were so arranged that their apices were turned toward the 

 mouth of the alveolus, i.e. in the direction from which the asci had 

 been illuminated during their development (Fig. 156). The asci on 

 the sides of an alveolus were curved outwards often through an 

 angle of about 45°, whilst those at the base of a chamber were 

 straight. Subsequently, using dried and pickled material (the former 

 cut with a hand-razor and the latter with the help of a microtome), 

 I confirmed Mr. Lowe's observations (Figs. 157 and 158). 



The curvatures of the asci in an alveolus of Morchella conica are 

 strictly comparable to the curvatures of the asci in the cup oiAleuria 

 vesiculosa or Galactinia badia, and there can be little or no doubt 

 that they are due to reactions to the hehotropic stimulus of Ught. 



A thin median- vertical section of a fruit-body of Morchella conica, 

 such as that shown in Fig. 157, A, not only permits one to perceive 

 the disposition of the hymenium within the alveoli, but also reveals 

 the very light physical framework which serves to support the 

 hymenium and to hold it in a position favourable for the discharge 

 of the spores : the flesh of the stipe and of the pileus roughly con- 

 stitutes a cyhnder, and the interior of the cyhnder is occupied by a 

 relatively very large air-space. Here, as in the stipes of many 

 Agaricaceae, e.g. Coprinus sterquilinus (Vol. IV, Fig. 66, p. 116), 

 and as in the stems of many Phanerogams, e.g. Gramineae and 

 UmbelHferae, the disposition of the supporting substance in the 

 form not of a soUd cylinder but of a hollow one increases the power 

 of the organ to withstand mechanical stresses and strains. 



On June 13, 1928, at Victoria Beach on Lake Winnipeg, Dr. G. R. 

 Bisby and I hunted the woods for Morchellaceae and found several 

 large fruit-bodies of Morchella crassijjes. With the help of a hand- 

 razor and a travelling microscope set up in the field-station of the 

 Manitoba Natural History Society, I examined the alveoh in cross- 

 section and found that, just as in those of M. conica, the asci were 

 curved towards the mouths of the alveoh. Apparently they had all 

 responded to the hehotropic stimulus of hght. Dr. Bisby, who was 

 good enough to examine my preparations, confirmed my observations. 



