PLANT COMMUNITIES AND FLORAS. 885 



for two or more species to remain sliarply marked off fi-om one another owing to the 

 fact of their flowering at different seasons. If tlie flowers of one species are ah'eady 

 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 

 aayngamy, is the cau.se whicli enables very similar species sometimes to live close 

 together without producing hj-brids, 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 Virgaiirea unfolds its eai-liest 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 another, 

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

 characters unaltered to their descendants. 



PLANT COMMUNITIES AND FLOEAS. 



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

 plant-species join together in communities ^, 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 gi-ouped 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. The 

 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 

 being of common origin, but in the nature of the habitat. They are forced into 

 companionship not by any afiinity to one another but by the fact that their vital 

 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 in 

 their requirements in re.spect of the intensity and duration of solar illumination, the 

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

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

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

 character of the rainfall. This does not, however, exclude the possibility that in 

 particular places similar demands may be met, and that difterent species with similar 

 needs may flourish undisturbed side by side as men live together in one house or in 

 one town, and, although their customs and their needs may not be exactly the same, 

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

 at home, because it rests upon common usages and is adapted to the local conditions. 

 Nor is it impossible that each one may derive an advantage fi'om the common Ufe, 



'C/. A. Kemcr von Marilaun " Gesterrcich-UugarDS Pflanzenwelt", in Die Oeilerrewhixk-UngarMte Monarchie 

 in WoH und Bild. Vol. i. p. 185 (1887). 



