PUFFING IN THE DISCOMYCETES 255 



What Factor determines the Oblique Position of the Opening 

 of each Ascus ? — It is known that the asci of many Ascobolaceae 

 and, as we shall see in the next Chapter, of many other Discomycetes 

 belonging to the Humariaceae, the Pezizaceae, and the Morchel- 

 laceae are positively heHotropic so that, during their development, 

 they bend their ends toward the Hght. The length of the part of 

 the ascus which may bend, relatively to the length of the ascus as 

 a whole, varies considerably in different species. Thus bending of 

 an ascus through 45° is effected in Aleuria vesiculosa by the outer 

 half of the ascus (Fig. 140, p. 293 ; cf. Fig. 123, A), but, in Ciliaria 

 scutellata, by no more than the last sixth of the ascus (Fig. 135, 

 p. 284 ; cf. Fig. 123, B). In all probabihty the ascus of Sarcoscypha 

 protracta, like that of so many other Discomycetes, is positively 

 heliotropic ; but, if so, its bending is confined to its extreme apex. 

 We have only to suppose that the operculum originates in the first 

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

 that, subsequently, the end of the ascus, after it has emerged at 

 the surface of the hymenium and has received unilateral light from 

 the mouth of the cup, makes a positively hehotropic curvature by 

 growing in length on its darker side just sufficiently to swing the 

 operculum round through an angle of about 45° until the operculum 

 comes to face the strongest incident rays of light directly (Fig. 123, C). 

 The assumption that such a heliotropic reaction takes place serves 

 to explain not only why the opercula of all the asci in the hymenium 

 look upwards toward the mouth of the cup {cf. Fig. 120, A and B, 

 p. 249) but also why they are obliquely situated on the ends of the 

 asci . Granted that the heliotropic reaction j ust suggested takes place, 

 then, in a ripe ascus, while the apparent axis is straight, the axis of 

 organisation is in reality curved at its extreme end (Fig. 123, C). 



Seaver ^ has supposed that the oblique position of the opening 



1 r. J. Seaver, The North American Cup-fungi, New York, 1928, pp. 17-18. 

 Seaver states that the apothecia of Phillipsia Chardoniana " are as flat as a pancake 

 yet, so far as observed, the ascostomes are always eccentric, or, at least, are con- 

 spicuously and predominantly so." However, he does not inform us whether or not 

 the apothecia under investigation were unilaterally lighted during their develop- 

 ment and whether or not the opercula all faced the source of greatest light. As will 

 be shown in the next Chapter, when the flat apothecium of Ciliaria scutellata is uni- 

 laterally lighted, the tips of all the asci bend toward the light, and the exploded asci 

 have opercula which are sub-oblique {cf. Fig. 135, 1 1, p. 284). 



