35 6 PLANT LIFE. 



rangium tends to straighten itself upon drying, thus rupturing 

 the sporangium. After bending backward for some distance 

 until the tear gapes wide, it suddenly straightens and hurls 

 the spores to a considerable distance (fig. 401). 



a b c 



Fig. 401. — Sporangia of the male fern (Aspidium Filix-»i a s) 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. 



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

 of spores depends solely upon their own motor organs. Only 

 zoospores (see ^j 306) 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. Even such relatively heavy spores as 

 the resting spores of algae may be carried long distances by 

 water currents. The microspores (pollen) of aquatic seed 

 plants are sometimes carried to the stigma by water currents, 

 as in Vallisneria (fig. 402). 



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

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



