THE PILOBOLUS GUN AND ITS PROJECTILE loi 



(p. 95) and 51 (p. 105), the width of the oval spot of light is about 

 equal to one- third of the width of the swelling. 



When the incident rays of light strike the sporangium and the 

 subsporangial swelUng head on and are exactly parallel to the 

 long axis of the swelling as shown in Fig. 46 

 (p. 91), the spot of light rests symmetri- 

 cally on the perforate protoplasmic septum 

 at the top of the stipe, is round in outline, 

 relatively small, and of maximum bright- 

 ness (Fig. 50, C and D). Its roundness is 

 due to the fact that the top of the swelling 

 through which the rays are refracted is 

 round. Its relatively small size and its 

 brightness are due to the fact that the rays 

 of light reach their focal point and begin 

 to diverge just above the protoplasmic 

 septum on which the spot of light is formed. 

 A study of the course of the rays of light 

 represented in Fig. 46 shows that at the 

 level jp in the particular fruit-body drawn 

 no direct rays pass through the main central 

 portion of the perforation of the septum, 

 so that a spot of light formed on a trans- 

 verse surface placed at the level of p would 

 have a small dark centre. It is of interest 

 to note that the length of the subsporangial 

 swelling is equal to, or only just exceeds, 

 its focal length, with the result that the 

 rays of light diverge before they reach the 

 perforate protoplasmic septum and so strike the concave surface 

 of the septum almost perpendicularly, i.e. in the most effective 

 manner for acting on the protoplasm and causing it to undergo 

 a photochemical change. The fact that the subsporangial swelling 

 is shaped like a pear is, therefore, from an optical point of view, 

 highly significant. 



For the further discussion of the nature of the spot of light 

 formed at the base of the subsporangial swelling of Pilobolus 



Fig. 50. — Pilobolus Klcinii. 

 Variation in shape of a 

 spot of light produced 

 by a beam of parallel 

 liglit rays in a subspor- 

 angial swelling. Light 

 in A transverse to axis 

 (cf. Fig. 51), in B 

 oblique {cf. Fig. 47), 

 and in C and D vertical 

 (cf. Fig. 4G). D, whole 

 of underside of .swell- 

 ing, stipe represented 

 by dotted circle. Mag- 

 nification, 38. 



