P8 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



indicating the humidity of the air, and the figures 

 are made to swing round by hygroscopic action. 

 This principle is utilised for the dispersion of plants 

 in a variety of ways. The simplest case is perhaps 

 the elaters attached to the spores of Equisetum. 

 Each spore is provided with four thread-like processes, 

 which curl and uncurl according to the varying dry- 

 ness or moistness of the atmosphere. The result is 

 that any change in the hygrometric condition of the 

 air causes the spore to move rapidly across a level 

 surface as if mounted on a liliputian bicycle. Elaters 

 of a slightly different form occur among the spores 

 of many species of Hepaticse. 



The hygroscopic property more commonly resides 

 in the sporangium, or spore-case, than in the spore 

 itself. The contraction of its walls while drying very 

 often causes a sudden rupture of the wall at its 

 weakest part, whereby the contents of the spore- 

 case are forcibly ejected. Ferns have an annulus 

 or ring of highly cuticularised cells surrounding the 

 sporangivim. This contracts strongly in drying, and 

 greatly assists in the expulsion of the spores. 

 Explosive anthers are common in wind-fertilised 

 flowers, such as Urtica, Parietaria, Plantago, grasses, 

 and others. This fact reveals the close analogy 

 between an anther and a sporangium. We may 

 even accept it as pretty certain that the wall of the 

 anther is in all cases more or less hygroscopic, 

 tending to open in dry weather and to close again in 

 wet, so that the pollen may be protected against 

 rain. 



The hygroscopic teeth on the peristome of Mosses 

 would appear to have a somewhat similar bearing 

 on the discharge of the spores. The dehiscence of 

 seed-capsules is brought about by the contraction of 

 their outer walls, and this often takes place in such 

 a way that the seeds are violently expelled. Thus 

 each half of the pod of broom {Sarothamnus) and of 

 the vetch {Vicia), at the moment of dehiscence, 

 suddenly twists round, whereby the seeds are 



