PLANT COMMUNITIES AND FLORAS. 885 



for two or more species to remain sharply marked off from one another owing to the 

 fact of their flowering at different seasons. If the flowers of one species are already 

 over when the other begins to bloom, no cross can take place between them under 

 natural conditions. This obstacle to cross-pollination, which has been termed 

 asyngamy, is the cause which enables very similar species sometimes to live close 

 together without producing hybrids, and thus prevents the origin of new inter- 

 mediate forms. For example, when Aster Amelliis begins to bloom the flowers of 

 the similar plant known as Aster alpinus are already over in the same locality, and 

 again, at the season when Solidago Virgaurea unfolds its earliest blossoms, the 

 flowers of the allied species Solidago alpestris, growing in the same neighbourhood, 

 have already set their fruits. Such asyngamic species, of which mention has already 

 been made on p. 510, are therefore found even in localities where their areas of 

 distribution are contiguous, and even where those areas dovetail into one anotli-T, 

 and where the various " petites especes " grow together and transmit their specitic 

 characters unaltered to their descendants. 



PLANT COMMUNITIES AND FLORAS. 



Wherever the reign of nature is not disturbed by human interference the different 

 plant-species join together in communities l , each of which has a characteristic form, 

 and constitutes a feature in the landscape of which it is a part. These communities 

 are distributed and grouped together in a great variety of ways, and, like the lines 

 on a man's face, they give a particular impress to the land where they grow. 

 species of which a community is composed may belong to the most widely different 

 natural groups of plants. The reason for their living together does not lie in their 

 beino- of common origin, but in the nature of the habitat. They are forced 

 companionship not by any affinity to one another but by the fact that their vit 

 necessities are the same. It may perhaps be true that amongst the many thousands 

 of plants inhabiting the earth no two are to be found which are completely alike i 

 their requirements in respect of the intensity and duration of solar illumination, t 

 concurrence of a particular duration of daylight with a certain amount of heat, t 

 composition and quantity of the nutrient salts available at the places wher 

 plants live, the amount of moisture in the air and in * ground, or 

 character of the rainfall This does not, however, exclude the possibility 

 particular places similar demands maybe met, and that different species with similar 

 needs may flourish undisturbed side by side as men live together in one hous, 

 one town, and, although their customs and their needs may not be exactl 

 yet form a society which is permanent and thrives, and wherein each member 

 2 to because It rests upon common usages and is adapted to the local cond.tK.ns. 

 Nor is it impossible that each one may derive an advantage from the , 



C/. A Kernarvon Marilaun "Oesterreich-Uugams Pfla^nwelt", i.. I"' 

 in Wort nnd Kid. Vol. i. p. 185 (1887). 



