230 STRUCTURAL BOTANY 



2. EEPRODUCTION 

 a. Asexual 



The most abundant means of propagation is by 

 asexual spores formed in sporangia. For the purpose 

 of spore-formation certain of the hyphse grow out into 

 the air, and assume a vertical position. The apex of the 

 aerial hypha enlarges into a nearly spherical sac, into 

 which most of the protoplasm travels from below. This 

 sac is cut off from the stalk by a transverse cell-wall, 

 and becomes the sporangium (see Fig. 96). Its contents 

 divide up simultaneously into a large number of round 

 cells, each of which surrounds itself with a cell-wall and 

 becomes a spore (see Fig. 96, E). We see, then, that in 

 this family the sporangium, instead of forming zoospores, 

 as in the last group, gives rise to motionless spores 

 with cell-walls. This is the typical method of asexual 

 spore-formation in this group. We will now consider 

 the special adaptations which are characteristic of our 

 type Pilobolus. 



The upper end of the stalk just below the sporangium 

 swells up into a bladder much larger than the sporangium 

 itself, and a second bladder is often formed in the lower 

 part of the hypha (Fig. 96, B). The lower of these two 

 bladders is separated by a transverse wall from the rest 

 of the hypha ; the upper is bounded above by the wall 

 which marks off the sporangium. The whole stalk 

 between the two transverse walls constitutes a water- 

 reservoir, in which a high hydrostatic pressure is set 

 up, so that drops of liquid often exude through the 

 membrane (Fig. 96, A, st). Owing to this pressure, the 

 wall bounding the sporangium becomes bulged into its 



