INVASION 215 
suriace by tensions within the apothecium. This might be regarded as dis- 
seminaticn 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 Gasteromycetes 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 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 Liguliflorae, 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. Seed 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- 
spermatae, Portulaca, Yucca, etc., in which 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, 
they 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. 
