INVASION 215 



surtace by tensions within the apothecium. This might be regarded as dis- 

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

 into the cup, unkss carried away by the wind. 



2. In Gasteromycetes and in certain Hcpaticac, 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 J^ryophyta, the sporophore regularly dehisces by a slit, or is pro- 

 vided 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 twisting and inter- 

 twining 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 Lignliflorae, Taraxacum, Agoseris, etc., are characterized 

 by the elongation of the scape after anthesis, with the result that the head is 

 raised to a considerable height by the time the achenes are mature. 



7. Carpotropic movements, though primarily for another purpose, often 

 serve to bring seeds and fruits into a better position for dissemination. 



266. 5eed production The relation of spore or seed-production to mobil- 

 ity is obvious in the case of mobile species; in the case of immobile ones, it 

 is just as evident that it has no effect, though it may still have considerable 

 influence in increasing migration. In the case of two species with equally 

 effective dissemination contrivances, the one with the largest seed-production 

 will be the more mobile. On the basis of the relation of seeds to flower, two 

 groups of plants may be distinguished, one, Polyanthae, in which the flowers 

 are many and the seeds few or single, as in Compositae, and the other, Poly- 

 spennatae, Portulaca, Yucca, etc., in w^hich the number of seeds to each 

 flower is large. So far as the actual number of seeds produced is concerned, 

 polyanthous plants may not differ from polyspermatous ones, but, as a rule, 

 ihey are much more highly specialized for dissemination and are more mo- 

 bile. The number of fertile seeds is also much greater, a fact which is of 

 great importance in ecesis, and which, taken in connection with mobility, 

 partially explains the supremacy of the composites. Among the fungi and 

 algae, the amount of spore-production in a large degree determines the mo- 

 bility, since these forms are intrinsically permobile. 



