Mosses, Ferns, and Horsetails. 



301 



The sporangia split open after the manner of anthers 

 and the spores are readily shaken out by the wind. The 

 spore is peculiar in having its outer coat split into four 

 ribbon-like bands 

 which coil and 

 uncoil with the 

 varying humidity 

 of the atmos- 

 phere. On dry- 

 ing, the bands 

 uncoil, and on 

 imbibing mois- 

 ture they coil up 

 again. Perhaps 

 the coiled bands, 

 when they come 

 in contact with 

 a suitable object, 

 may serve to an- 

 chor the spores 

 in a moist place 

 where the con- 

 ditions for ger- 

 mination are 

 good ; or in dry 

 situations, where 

 the bands are spread out, they may serve as sails to 

 catch the wind so that the spores may be carried to 

 more favorable situations. The bands may also prove 

 useful in binding several spores together ; for since the 

 spores on germinating give rise to prothallia bearing 

 only eggs or sperms, the prothallia being dioecious, it 

 would be obviously in the interest of the fertilization 



FIG. 160. 



Equisetum arvense. C, a sterile shoot ; D, fertile shoots ; 

 o and p, clusters of sporangia ; q and r, sporophylls 

 bearing sporangia ; s, f, and u, spores, with spiral bands 

 uncoiled at t and u ; v, tubers on the underground stem. 

 After WOSSIDLO. 



