

PHOTOTROPISM 131 



that, since air is a less dense medium than is the content of the 

 sporangiophore, the rays of light refracted from the front half of 

 the cylindrical cell converge on the side opposite the source of 

 light. Similarly, the oil is a more dense medium than is the con- 

 tent of the sporangiophore, and hence the rays of light after 

 refraction diverge from one another. When air is the medium, 

 the back half of the growing zone is lighted the more intensely; 

 when oil is the medium, the front half. The growth response is 

 therefore in opposite directions in the two cases. 



Castle (1933) has also contributed to an understanding of the 

 response of P. nitens to light. His solution of the problem is 

 based upon three assumptions: (1) bending is a resultant of un- 

 equal absorption of light by the two halves of the cell (the half 

 toward the source of light and the half most distant from the 

 source); (2) the primary action of light is upon the protoplasm; 

 and (3) the absorption of light is brought about by a substance 

 or substances (pigment) equally distributed within the cell. From 

 these reasonable assumptions he deduces that the factors which 

 govern the unequal action of light in the two halves of the cell 

 are the following: (1) the refractive index of the cell, (2) the 

 size of the cell, more specifically its radius, and (3) the coefficient 

 of absorption possessed by the intracellular pigment. 



All known species of Pilobolus, which commonly occur on 

 the fresh dung of herbivors when it is kept in a moist chamber, 

 exhibit photic reactions. Among those who have investigated 

 the response of these species to light are Allen and Jolivette 

 (1914), Parr (1918), Pringsheim and Czurda (1927), van der 

 Wey (1929), and Buller (1934). Allen and Jolivette admitted 

 light through a pinhole and found that Pilobolus aimed point- 

 blank at the light. The accuracy of the aiming was remarkable, 

 for when the culture was 20 cm distant from the opening, 95% 

 of the sporangia struck within a ring 4 cm in diameter, the remain- 

 ing 5% being within the next 1 or 2 cm. A greater degree of 

 precision was obtained with white or blue light and less accuracy 

 with yellow light; the aiming was very inaccurate with red light. 

 Of course the distance that the sporangia needed to travel modified 

 the precision. Allen and Jolivette also made the interesting obser- 

 vation that, when Pilobolus was exposed to two equal beams of 

 white light with the angle between them greater than 10° of arc. 



