GOLD COAST REPORT ON FORESTS. 167 



may be. When these condition* alternate one with another with 

 a regular periodicity, we get the third or tropophilous type. 



The boundaries between these divisions are, however, not 

 sharply marked in nature, and intermediate conditions are often 

 met with. Those mentioned, however, correspond to the maxi- 

 mum adaptations acquired under pronounced conditions of 

 physiological wetness or dryness, and their periodical alterna- 

 tions. 



This relation of plants towards water is expressed by structural 

 characters ; where the latter is such as to favour the exit of the 

 water absorbed by the roots we get the class known as Hygro- 

 phytes, and where it retards the exit of moisture, the class termed 

 Xerophytes ; Tropophytes are plants the structure of whose 

 perennial parts is that of Xerophytes, and that of their parts 

 (leaves) present only in the wet season that of hygrophytes. 



Wherever moisture is abundant, and, therefore, the danger 

 of desiccation absent, plants have, in order to prevent any possi- 

 bility of stagnation in the water-current (transpiration current) 

 that distributes nutritive salts to the various parts requiring 

 them, acquired .contrivances that hasten the exit of water; 

 such ulants are Hygrophytes. 



Where, on the other hand, the supply of moisture is scanty 

 and the strength of the transpiration current relatively to the 

 water supply has to be regulated so as to guard against physio- 

 logical dryness, the corresponding adaptations have brought 

 about the plants known as Xerophytes. 



Hygrophytes have a weakly-developed root system, elongated 

 axes, large thin leaf-blades, and they but rarely develop thorns 

 (not prickles). In moist tropical forests with a heavy rainfall, 

 the plants often develop leaves with a long " dripping point," 

 and the leaves frequently have a velvety surface on which the 

 water spreads out into an extremely thin layer that rapidly 

 evaporates. The leaves, again, possess well-developed air-con- 

 taining intercellular spaces that strongly promote transpiration, 

 and they are, especially in the case of trees forming the moist 

 tropical evergreen forests, rich in Hydathodes or organs for 

 excreting liquid w r ater. The heavy showers of the so-called 

 " dew-drops," that are so often noticed late at night and in the 

 early hours of the morning, to drip off tropical forest trees in a 

 moist locality, mainly consist of water excreted by these 

 Hydathodes. 



Finally, Hygrophytes, are to a large extent Ombrophilous or 

 rain-seeking, and their foliage is capable of being wetted. 



Xerophytes, on the other hand, have to accommodate them- 

 selves to conditions that involve the danger of physiological 

 dryness. This may be brought about by increased transpiration 

 (with respect to the available water supply) due to the following 

 causes: A dry atmosphere, reduced atmospheric pressure (rare- 

 faction of the air), and strong light, or by reduced absorption due 

 to the scarcity of free water in the soil, excess of soluble salts 

 or humous acids in the soil, or to a low temperature of the soil. 



