ORDER MUCORALES 153 



occupy and does not represent a cross wall that subsequently bulged up 

 into the sporangium. This gives a much larger surface to the septum and 

 permits a much freer transfer of food into the sporangium. Accompanying 

 this more efficient food supply we find the sporangia to be larger, almost in 

 proportion as the columella increases in size. In these two types of spo- 

 rangium the numerous spores escape by the dissolution or breaking up 

 of the relatively thin sporangial wall. The columella often remains firm 

 and unchanged after the sporangium has ruptured and the spores have 

 been set free. Schostakowitsch (1896) reported that in Meteor proliferus 

 Schost. the columella forms a new sporangium by proliferation, reminding 

 one of the condition in Saprolegnia. (Figs. 49 A, 50A-C.) 



The sporangium of Pilobolus represents a special modification of the 

 foregoing type. In it the apical wall of the many-spored sporangium is 

 very much thickened. The columella is rather small. Below the spo- 

 rangium the sporangiophore is enlarged into a subsporangial vesicle that 

 may be two or three times the diameter of the somewhat flattened 

 sporangium. The sporangiophore tip is sensitive to light and this leads 

 to the curvature of its lower part so that the sporangium is directed 

 toward the source of the light. As the vesicle enlarges the turgor finally 

 becomes so great that the apex ruptures and the sporangium is blown off, 

 along w4th the watery contents of the vesicle, sometimes to a distance of 

 over a meter. The sporangia adhere to vegetation and are eaten by 

 herbivorous animals through whose digestive tracts the spores pass un- 

 harmed. In the dung of these animals the mold grows and produces its 

 conspicuous fructifications. (Figs. 51A, 52.) 



Returning to the genus Mucor we find that some species have un- 

 branched sporangiophores while in others the sporangiophores may 

 branch sympodially or monopodially, each branch terminating in a spo- 

 rangium. Usually these sporangia are approximately equal in size al- 

 though frequently the terminal one is slightly larger. In M. prolifer-us 

 and some other species, the spores produced in the lateral sporangia are 

 smaller than those produced in the larger, terminal sporangium. In 

 Thamnidium the terminal sporangium, which is often the first one formed, 

 is larger and possesses a well-developed columella; somewhat below it 

 there grow out from the sporangiophore short branches (often much 

 forked dichotomously), all terminating in small sporangia or some in 

 pointed spines. These small sporangia (sporangioles) are few-spored, 

 sometimes with not over two or three spores, and possess no columella. 

 The whole sporangiole becomes detached and distributed by air currents. 

 As water is absorbed the spores swell and burst the sporangial wall and 

 escape. In some species of Thamnidium, under certain conditions only 

 the sporangioles are formed. In the genus Dicranophora the sporangioles 

 are only one- to two-spored and their spores are much larger than those 



