THE LIFE-FORMS OF RAUNKIAER 175 



carbon dioxide and of undecayed or only partly decayed organic 

 matter. One result of this is a rather high degree of acidity of the 

 water. Possibly some other substances which may be somewhat 

 toxic to the roots of plants also accumulate. The habitat is not well 

 understood but it seems to be difficult for plants to absorb water 

 from it and undoubtedly the acidity is one of the factors causing this. 

 The peat moss {Sjihagnnm) is especially characteristic of bogs. 

 There are also many orchids, insectivorous plants, and such woody 

 plants as cranberry (Vaccinimn), leather leaf {Chamoeda'phne) , bog 

 rosemary (Andromeda), and tamarack trees {Larix laricina). Many 

 of these plants, especially the woody ones, show marked xeric 

 features. Some of them, the tamarack for example, like many 

 plants of saline habitats, grow better elsewhere if given a chance, 

 indicating that they "tolerate" rather than "prefer" these physio- 

 logically dry habitats. This means only that, under ordinary condi- 

 tions, they are not able to compete with other plants in more favor- 

 able habitats but are able to meet competition in the unfavorable 

 ones. 



110. Plants of Mesic Habitats.— The most typical mesic habitats 

 are found in deciduous forest regions. In many respects the struc- 

 tural characteristics of mesophytes are more or less strictly inter- 

 mediate between those of the xerophytes and hydrophytes that we 

 have discussed, but, of course, all degrees of gradation could be 

 found between the few typical habitats with which we are dealing. 



In the typical mesic habitats there is usually a prominent develop- 

 ment of vertical roots with abundant root hairs. The foliage reaches 

 a maximum development and the leaves are relatively large and thin. 

 The thin, transparent layers of epidermis and an abundance of 

 chlorophyll cause the lea;ves to appear dark green, in contrast to the 

 pale green leaves characteristic of xeric plants. Stomata usually 

 occur on both surfaces of the leaves, except in the case of trees where 

 they ordinarily are on the lower surface only. The guard cells have a 

 maximum capacity for movement and cutinization of walls is usually 

 moderate. 



111. The Life-forms of Raunkiaer.— Raunkiaer is a Danish 

 botanist. His classification of plants into life-forms is the result of 

 an attempt to find some method of estimating the relative values of 

 the climates of various parts of the world by means of some standard 

 that would have a direct bearing on plant life. The ordinary physical 

 methods of measuring climatic influences are unsatisfactory because 

 very different physical factors may have quite similar effects on 



