226 



SPORANGIUM OF PHILOBOLUS 



in this mode of scattering the spores is seen in a related form, 

 Pilobolus, which is of common occurrence upon horse dung. 

 Here the wall of the sporangium, unlike Rhizopus, is quite firm 

 and becomes mucilaginous only at the point of contact with the 

 stalk that bears it. Owing to the accumulation of water in the 

 stalk such a pressure is finally set up as to rupture it at the 

 mucilaginous point and so the sporangium with its contained 

 spores is hurled considerable distance— often quite one meter. 

 The sporangia bearing stalks are also sensitive to light. If 

 a glass jar covered with black paper so as to exclude all light is 



Fig. 136. Sexual reproduction in the black mould: A, the meeting of 

 the tips of two hyphae. B, later stage, the lower part of the figure shows 

 the cutting off of the tips by transverse walls and in the upper part of the 

 figure the fusion of the contents of the two gametangia thus formed has be- 

 gun. C, mature gametospore. 



placed over a colony of these plants and a minute opening is now 

 made in the paper, you will be surprised to note with what 

 accuracy each sporangium is shot off and hits this opening. 

 The results are the same no matter where you make the opening. 

 Under certain conditions the sexual method of reproduction is 

 effected by the union of two club-shaped hyphae, as shown in 

 Fig. 136, A. As these hyphae meet, the tip of each branch is 

 cut off by a wall and the contents of the two tips fu.se, forming a 



