THEPILOBOLUS GUN AND ITS PROJECTILE 93 



light (Fig. 46) and (2) when its axis is inchned to the direction of the 

 incident rays at an angle of 19° (Fig. 47). 



In constructing Figs. 46 and 47, an outline of a typical fruit- 

 body of Pilobolus Kleinii was first very carefully drawn with the 

 camera lucida. Then the parallel hues Nos. 1, 2, 3, 4, etc., were 

 added to represent the paths of a few rays of light directed toward 

 the fruit-body. Each ray was now treated separately : its angle 

 of incidence at the surface of the subsporangial swelling or stipe 

 was measured with instruments ; its angle of refraction into the 

 swelling or stipe was then calculated ; and, finally, the angle of 

 refraction being known, a line representing the path of the ray 

 through the interior of the subsporangial swelling or stipe was 

 drawn. In calculating the paths of the refracted rays of light, the 

 following refractive indices were used : air into cell-sap, ^ 1 • 34 ; air 

 into cell- wall and protoplasm taken together,^ 1 • 5 ; cell- wall and 



Fig. 47 — cotit. 



it is somewhat thickened and at p where it bulges inwards to form a large 

 biconcave perforate septum containing red particles massed especially near its 

 \]pper surface : the stipe ; the large clear vacuole enclosed by the protoplasm 

 of the stipe, subsporangial swelling, and columella ; and the abscission line a. 



Twenty-three parallel rays of light are shown diagrammatically, by means of 

 arrows, all inclined at an angle of 19" to the axis of the subsporangial swelling. 

 Of these rays : eight strike the black cell-wall of the sporangium, which they 

 cannot penetrate ; eight strike the right-hand wall of the subsporangial 

 swelling and are there refracted through the wall into the interior of the swelling, 

 so that, as shown by the arrows, they converge to form a spot of light on the 

 left-hand wall at st ; and six strike the right-hand wall of the stipe and are there 

 refracted as shown by the arrows, s, s, s are areas in the subsporangial swelling 

 which are not pierced by any of the rays of light and which therefore are in 

 the shade. The concentration of the rays of light at the spot s< presumably 

 gives a photochemical stimulus to the protoplasm, which stimulus is conducted 

 down to m, the motor region of the stipe. The stipe grows faster on the side 

 TO than on the opposite side, with the result that the subsporangial swelling 

 and the sporangium are turned about the top of the stipe through an angle of 

 19-", i.e. until their axis becomes parallel to the direction of the incident rays 

 of light. As the subsporangial swelling and sporangium are turned, the spot 

 of light st descends to the base of the swelling until, finally, it comes to rest 

 symmetrically upon the red perforate protoplasmic septum, as shown in Fig. 46. 

 Heliotropically (or photochemically) the position of stable physiological equi- 

 librium has then been reached and no more turning takes place. The part of 

 the stipe below the motor region r«, at and below u, does not alter its position 

 whilst the motor region is making its curvature. The scale serves to indicate 

 the size of each part. Magnification, 69. 



1 The cell-sap was regarded as pure water. The refractive index from air to 

 water is 1-34. 



2 The refractive index 1 • 5 here used is the same as that found by Senn for the 

 refraction from air into the cell-wall of Vaucheria, etc. Vide G. Senn, Die Gestalts- 

 und Lageverdnderung der Pflanzen-Chromatophoren, Leipzig, 1908, pp. 363-366. 



