GOLD COAST REPORT ON FORESTS. 169 



(4.) Sea-shores and some soils in which the supply of salt 

 is abundant. 



In all these habitats physiological dryness is a standing- 

 danger, and only Xerophytes can successfully occupy them. 



Tropophytes. 



Plants occupying a habitat that is alternately hygrophilous 

 and xerophilous in climate, have adapted their structure to these 

 periodically different conditions, and are known as tropophytes. 

 Such a climate is one in which a season of fairly abundant rain- 

 fall is invariably followed by a well-marked dry period of the 

 year. Immense tracts of the earth's surface experience a climate 

 of this sort, and it is immaterial to the plant whether the 

 periodical dryness is due to a scarcity of atmospheric precipita- 

 tion or to a low temperature of the soil, the latter condition 

 being just as capable of inducing physiological dryness as the 

 former. Hence, large portions of the Temperate Zones, where 

 severe winters prevail, are occupied by Tropophytes as well as 

 extensive areas in the Tropical Zone. 



The difficulty of adapting the structure of plants so as to 

 satisfy the requirements of these, two very dissimilar periods of 

 the year has been met by their perennial parts, such as the 

 stems, branches, roots, &c., being modified to withstand xero- 

 philous conditions, and their temporary parts, the leaves, being 

 adapted to hygrophilous conditions such as prevail during the 

 wet season. When the dry period of the year comes round and 

 transpiration has to be reduced, the leaves are shed, a simple 

 and effective device to guard against all possibility of danger 

 from excessive transpiration during the season of phj'siological 

 dryness. 



Characteristics of Tropopliytes. 



(1.) They sacrifice (by shedding their leaves) the greater portion 

 of their transpiring organs at the beginning of the physiologic- 

 ally dry season, whether the latter is brought about by a dry 

 climate or a cold one. Most of the woody plants shed their 

 leaves at this season. Many herbaceous plants lose all their 

 sub-aerial parts and merely retain their subterranean ones, which 

 transpire but slightly. 



(2.) Periodically foliaged tropophilous woody plants have 

 hygrophilous leaves, but xerophilous axes (stems, &c.) and buds. 

 The stems and branches are protected against drought by a 

 well-developed bark or thick layers of cork, and the buds by 

 hard and often lacquered scales. 



(3.) Evergreen tropophilous plants have xerophilous foliage as 

 well as xerophilous axes and buds. 



The most obvious characteristic is the periodical foliation and 

 defoliation which in some localities is most conspicuous, as the 

 foliation is often very luxiiriant during the wet season, and in' 

 marked contrast with the gaunt, bare-stemmed habit assumed 

 during the dry season. 



