1G6 PHYCOMYCETEAE 



small window, the sporangiophores can be induced to turn and discharge 

 their sporangia in the direction of the window, without much scattering 

 of aim, if the illumination is good. Miss R. F. Allen and Miss H. D. M. 

 Jolivette (1913) and later Miss Jolivette (1914) studied the relation of 

 the discharge to different colors of light qualitatively. Miss Parr (1918) 

 studied these relations quantitatively. The light at the violet end of the 

 spectrum is the most efficient, grading to the red without intermediate 

 maxima or minima. The presentation time, i.e., the minimum length of 

 exposure necessary to bring about the effect, "varies in inverse ratio to 

 the square root of the wave frequency" (Parr). Flint (1942) demonstrated 

 that an unidentified species of Piloholus gave no curvature but abundant 

 discharge of sporangia in random directions in red light of the range of 

 6576-7024 A units, and strong phototropic curvature and discharge with 



o 



blue light centering at 4600 A units. 



As the sporangiophore is elongating, before the sporangium has begun 

 to form, it is pointed and positively phototropic, so that it points toward 

 the direction of the strongest light of effective wave length. With the 

 formation of the subsporangial vesicle the latter has been shown by 

 Buller (1921, 1934) to form a very effective light-perceiving organ or 

 "eye" which brings about the final aiming so that the sporangium is shot 

 off in the most favorable direction. Buller showed that in the vertical 

 direction the sporangium may be shot over six feet and horizontally, 

 eight feet. 



In this family the suspensors of the gametangia are curved like a pair 

 of tongs. 



Grove (1934) recognizes 9 species oi Piloholus with 7 additional doubt- 

 ful species, and 5 species of Pilaira, while Naumov (1939) recognizes 

 respectively 18 and 5. (Figs. 51, 52.) 



Family Thamnidiaceae. The genera of this family show the begin- 

 ning of the differentiation into two types of sporangium, viz., the large 

 terminal sporangium with a columella and the small lateral separable 

 sporangioles which lack a columella. The latter are borne on short, var- 

 iously divided branches arising laterally on the main axis of the spo- 

 rangiophore at whose apex the large sporangium is produced, if present. 

 In Thamnidium the spores are alike in sporangium and sporangiole. In 

 some species some of the branches upon which the sporangioles are borne 

 are sterile at the tips and form spine-like processes. The sporangiole- 

 producing branches are dichotomously forked. The zygospores are formed 

 on approximately equal suspensors about as in Mucor. In Helicostylum 

 the sporangioles are borne on short circinate branchlets from the un- 

 forked lateral branches. In Dicranophora the sporangioles, according to 

 Dobbs (1938), are one- to few-spored and have a rounded columella and 

 thin sporangial wall. The spores are very variable in size and mostly 



