Chap. I] 



WATER 



21 



water-stomata, and so forth. They are sometimes active glands com- 

 parable to sweat-glands, sometimes passive places of exit associated with 

 a simple process of filtration. In very damp regions hydathodes are often 

 very numerous. Thus Haberlandt found, on an average, fifty-five hyda- 

 thodes per square millimeter on the upper surface of a leaf of Gonocaryum 

 pyriforme, and fifty-eight on an equal area of its lower surface. 



Many other features in the more minute structure of hygrophytes, such 

 as the red and silver spots on variegated leaves, have been regarded 

 as favouring the exit of water. We must leave it to further research 

 to discover how far the ingenious and suggestive explanations of these 

 phenomena are borne out by facts l . 



iv. TROPOPHYTES. 



The vegetation of districts with climates alternately damp and dry or 

 cold, is alternately of a hygrophilous and of a xerophilous character ; 

 it is therefore tropophilous. The equivalence of cold and dry seasons as 

 regards the supply of water to plants 

 has caused similar adaptations in both 

 cases. 



Most tropophytes, whether of an 

 alternately dry and moist climate or 

 of an alternately cold and hot one, 

 sacrifice the greater part of their 

 transpiring organs at the beginning 

 of the physiologically dry season. 

 Many herbaceous plants lose all their 

 subaerial parts, and merely retain 

 their subterranean ones, which trans- 

 pire but slightly. Others retain only 

 the leafy shoots near the ground, in 

 the form of rosettes or otherwise 

 grouped. Most woody plants shed their leaves. 



Periodically foliaged tropophilous woody plants have hygrophilous leaves, 

 but xerophilous axes and buds. Stems and branches are protected against 

 drought by bark or by thick layers of cork, buds by hard and often 

 lacquered scales. In the case of evergreen tropophilous woody plants, 

 xerophily must extend to the foliage, or the latter would perish from want 

 of water in the dry or cold season. Such tropophytes are therefore, 

 except for their young shoots, entirely xerophilous in construction ; they are, 

 however, distinguished from xerophytes by their environment. Examples 

 from our own flora are the silver-fir and the spruce (not the really xerophilous 

 Scots pine of dry sandy soil), holly (Fig. 28), cowberry, heather, and others. 



1 See Stahl, IV. 



Fig. 28. Xerophilous structure in the peren- 

 nial leaf of a tropophilous plant. Ilex Aqui- 

 folium. After Stahl. 



