DISTRIBUTION OF SPORES AND SEEDS. 273 



After bending backward for some distance until the tear 

 gapes wide, the spring suddenly straightens and hurls the 

 spores to a considerable distance (fig. 236). 



A B C 



Fig. 236. — Spore cases of the male fern {Aspidium Filix-mas) scattering the spores. 

 A , closed ; B^ burst by the drying of the annulus ; C, the annulus after becoming 

 strongly recurved is just returning to a nearly straight form and the spores are thereby 

 being hurled toward B. Magnified about 65 diam. — After Kerner. 



385. 2. By water. — In perfectly quiet water, distribution 

 of spores depends solely upon their own motor organs. Only 

 zoospores (see T[ 264) are so furnished. For these a film of 

 water is sufficient, and they may swim some distance over 

 what appear to be merely moist surfaces. Most of the algae 

 and fungi living in water form zoospores. Their production 

 is often controlled by external conditions, the formation of 

 new individuals being thus provided for when the old are 

 threatened with destruction. 



In flowing water and by currents, non-motile spores are 

 readily distributed, and even relatively heavy spores may be 

 carried long distances by water currents. The pollen of 

 aquatic seed-plants is sometimes carried to the stigma by 

 water currents, as in Vallisneria (fig. 237). 



386. 3. By air currents. — Spores may be readily carried 

 by the air on account of their small size and their ability to 

 withstand dryness. Most spores float in the air for some 

 time as dust particles, and the slightest current is adequate 



