814 



THE DISTRIBUTION OF SPECIES BY OFFSHOOTS. 



teeth, each of which terminates in a free point. The tissue of these teeth is hygro- 

 scopic, and their direction and position alter to a surprising extent according to the 

 degree of humidity of the air. In damp weather the teeth are so close together 

 that they completely shut the capsule (fig. 450 9 ), but in dry weather they bend 

 outwards (fig. 450 10 ), and the spores are shaken out of the capsule and scattered by 

 the wind. 



We shall have to describe presently how the sporangia of most Ferns dehisce 



Fig. 450. Dispersal of spores by wind. 



' fn d" ^ 7vT \ %?* *"*' ' The 8ame in dry Weather " 8 Spore-capsule of a Polytrichum in damp 

 i W ? *, 8p re - Ca P sule of a Polytrichum, the edge of the mouth beset with teeth and covered 



' SoZTf "^ *"***' ' A ^ f the Perist me more hi hly ma = nifled ' in dam P Bather. 

 fin dry ^^ * ^ f***' " Ia dry Weather ' " Racemose sporan ia of a 

 dry "^ * 



suddenly so as to scatter the spores. In such Ferns the sporangia are developed on 

 the under side of the frond, and this position protects them excellently against any 

 injury which might befall them from rain or dew. But there are some Ferns whose 

 sporangia are exposed to both rain and dew, and whose spores are not suddenly 

 scattered by the bursting of the sporangia. Among others, the Moonwort 

 r?L% menti0ned ' ^ branChed S P ike f S P ra ^ ia is ^presented in 



iX; ? el !! ptical sporangia of the koonwort openb ^ a trans - 



it, but the two valves thus formed only separate in dry weather (figs. 450 



