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^), but in dry weather they bend 

 outwards (fig. 450 ^''), 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. 



1 Spore-capsule of an Andrecea in damp weather. 2 xhe same in dry weather. » Spore-capsule of a Polytriehum in damp 

 weather. 4 In dry weather, s The spore-capsule of a PolytHchuTn, the edge of the mouth beset with teeth and covered 

 with a membrane, in damp weather. 6 In dry weather. 7 a part of the peristome more highly magnified, in damp weather. 

 » In dry weather, s Spore-capsule of a Grimmia, in damp weather. 10 In dry weather, n Racemose sporangia of a 

 Botrychium in damp weather. 12 in dry weather, is a single sporangium of this Botrychium enlarged, side and front 

 view, in damp weather. 1* In dry weather. », *, " and 12 nat. size ; the others enlarged. 



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 

 (Botrychium) may be mentioned. Its branched spike of sporangia is represented in 

 figs 450^^ and 450^^. The elliptical sporangia of the Moonwort open by a trans- 

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



