MIGRATION 43 



1. In all operculate Discomycetes, and especially in the 

 Ascobolaceae, where the asci project above the hymenium, 

 the spores are raised above the surface by tensions •within 

 the apothecium. This might be regarded as dissemination 

 by expulsion, if it were not for the fact that the spores fall 

 back into the cup. unless carried away by the wind. 



2. In Oasteromycetes and in certain Hepaticae. the spores 

 are not only elevated slightly above the sporophore by the 

 expanding capillitium or by the mass of elaters, but they 

 are also held apart in such a way that the wind blows them 

 out much more readily. 



3. In Bryophyta, the sporophore regularly dehisces by a 

 slit, or is provided with a peristome. Both structures are 

 for the purpose of sifting the spores out into the wind : by 

 reason of their hygroscopicity, they also insure that the 

 spores will not be shaken out in wet weather. 



4. In a few grasses, such as Stipa and Aristida, the twist- 

 ing and inter wining of the awns lift the floret out of the 

 glumes, and at the same time constitute a contrivance 

 readily blown away by the wind or carried by attachment. 



5. In certain Compositae, the involucral scales are reflexed 

 at maturity, and at the same time the disk becomes more or 

 less convex, serving to loosen the achenes. This result is 

 also secured in certain species by the drying and spreading 

 of the pappus hairs. 



6. The scapose Liguliflorae, Taraxacum, Agoseris, etc., are 

 characterized by the elongation of the scape after an thesis, 

 with the result that the head is raised to a considerable 

 height by the time the achenes are mature. 



7. Oarpotropic movements, though primarily for another 

 purpose, often serve to bring seeds and fruits into a better 

 position for dissemination. 



The relation of spore or seed-production to mobility is 

 obvious in the case of mobile species : in the case of im- 

 mobile ones, it is just as evident that it has no effect, though 

 it may still have considerable influence in increasing mi- 

 gration. In the case of two species with equally effective 

 dissemination contrivances, the one with the largest seed- 



