CHAP, xxxiv OECOLOGICAL CLASSES 135 



clothed with forest. 1 Among the formations belonging to this type 

 must be reckoned those forming steppes and savannahs, also certain 

 sclerophyllous formations. 



Mesophytes grow on soil which is of an intermediate character, and 

 is neither specially acid, cold, nor saline, but is moderately moist, usually 

 well- ventilated, also rich in nutriment and in alkaline humus or in other 

 organic constituents. Mesophytic communities occur in very diverse 

 climates, near the Poles or on the equator, yet they can never be exposed 

 to the danger of prolonged drought. Adapted to such conditions are 

 plants that show a relatively weak development of the above-mentioned 

 arrangements for regulating transpiration ; in this respect these plants 

 stand midway between hydrophytes and xerophytes. The leaves are 

 large, and far more varied in form than in xerophytes ; teeth and other 

 incisions of the margin are common, as are compound or richly divided 

 leaves ; hydathodes seem to be frequent ; the vegetative organs are of 

 a fresh green, and devoid of thick grey coatings of hair or bluish incrusta- 

 tions of wax ; the leaves are usually dorsi- ventral in structure. Stomata 

 are numerous, often occurring also on the upper face of the leaf ; anatomi- 

 cal peculiarities, such as aqueous tissues, are very rare, or at least not 

 extremely developed. 



The greatest differences among mesophytes depend upon whether 

 the leaves persist for only a few months in the favourable season, or for 

 a year or more. Ilex Aquifolium is indubitably a mesophyte growing 

 as underwood in forests of northern Europe ; but its leaves persist for 

 up to two years, and, like sclerophyllous leaves, are xerophytic in structure 

 because they are exposed to the harsh conditions of winter that is, to 

 cold (physiologically dry) soil and possibly concurrent rapid transpiration 

 caused by dry, cold winds ; the same is true of the spruce (Picea excelsa) 

 and most other evergreen woody plants in cold-temperate countries. 

 In deciduous plants within the same countries the leaves are thinner, of 

 a paler green, and more flexible ; the cuticle is thinner, and so on ; they 

 are, in short, typically mesophytic in structure. 



In the tropical rain-forest, which may be regarded as a mesophytic 

 community, there are many species possessing leaves that are xerophytic 

 in structure because they persist for more than one year, and must 

 consequently be adapted to endure all the changes during that period. 

 It is also difficult to regard all conifers as xerophytes, even when their 

 leaves are perennial. 2 



Pound and Clements 3 divide mesophytes into three groups : hylophytes (woody 

 plants), po-ophytes (meadow plants), and aletophytes (ruderal plants). But it 

 must be noted that there are forests and grasslands among xerophytic formations. 



Between the different groups there are .very gradual transitional stages. 

 Moreover, the peculiarities of a formation may be evoked by a com- 

 bination of diverse factors of varied strengths ; for instance, there seem 

 to be formations for the origin of which the co-operation of coldness and 

 acidity of soil is responsible. 



Tropophytes. Schimper * has introduced the term ' tropophyte ', by which he 

 designates land-plants which, in opposition to hygrophytes and xerophytes, have 



1 Warming, 1892, 1899. * See Section XV. 



:l Pound and Clements, 1898; Clements, 1904, p. 22. * Schimper, 1898. 



