232 STRUCTURAL BOTANY 



oxalate, while the lower part adjoining the columella 

 remains thin. Inside the lower part of the sporangium 

 is a mucilaginous layer derived from its protoplasm ; 

 when the sporangium is wetted, this mucilage takes up 

 water and swells, bursting the delicate cell-wall, and so 

 freeing the upper portion of the sporangium which 

 contains the spores (Fig. 96, F). The pressure on the 

 upper surface of the columella is thus removed, and no 

 longer balances that of the liquid in the reservoir below ; 

 consequently the columella bursts, and a jet of water 

 escapes from the reservoir, driving the sporangium and 

 spores violently before it (Fig. 96, D). The sporangium 

 may thus be hurled to a great distance, amounting 

 it is said to more than a yard in some cases. Hence 

 the name of the plant, which means " a thrower of 

 missiles." The sporangium sticks to any object which 

 it happens to hit, owing to the mucilage which still 

 clings to it. When the spores from the sporangium 

 germinate they reproduce the ordinary form of the 

 Pilobolus plant. 



Other modes of propagation have occasionally been 

 observed. The normal sporangia are only formed in air ; 

 if, however, the mycelium is forced to grow in a liquid 

 containing plenty of organic food material, another 

 process takes place. The hyphse divide up by trans- 

 verse walls into numerous cells, which may increase in 

 number by budding, each cell putting out a short branch, 

 which becomes separated from the parent. This is called 

 the Oidium condition. In other cases, namely when food 

 is less abundant, some of the cells produced by transverse 

 division of the mycelium may acquire thick walls, and 

 pass into a resting condition. These thick-walled cells 

 are called chlamydospores, and, like the oidiospores, may 



