30 BKITISH PLANTS 



approach of winter, and are renewed in the spring, the 

 plants to which they belong cannot be regarded as true 

 hygrophytes (see Tropophytes, p. 57). 



2. Xerophytes. 



Most plants have to face a deficiency of water some- 

 times, and if there were no peculiarity of habit or 

 structure to prevent them from suffering during these 

 periods, their existence would be threatened, and the 

 continuance of the race endangered. Every peculiarity 

 of habit and every peculiarity of structure or mode of 

 growth which enables a plant to get through a period 

 when water is lacking either in quantity or quality is 

 known as a xerophytic character, and these characters 

 may be few or many, slight or pronounced, permanent 

 or temporary, according to the conditions which obtain 

 in the normal surroundings of the plant. 



Just as the true hygrophyte exhibits adaptations of 

 a permanent character towards moist conditions, so a 

 true xerophyte shows adaptations of a permanent 

 character towards dry. The unfavourable conditions 

 may only come once a year, and if the plant meets this 

 by some permanent modification in its structure it is 

 a true xerophyte. Thus the holly is common in moist 

 woods in the West of England. Its leaf is evergreen, 

 thick, and shiny characters associated with deficiency 

 of water (p. 39). The deficiency, however, only occurs 

 in winter, when, through the coldness of the soil, the roots 

 become inactive and lose, more or less completely, their 

 power of absorption. The holly therefore exhibits 

 during the summer characters which are only really useful 

 in winter. In other cases, unfavourable conditions occur 

 all the year round ; during summer they may be of one 

 kind, during winter of another. When this is so, we 

 should naturally expect the plants to be equipped with 

 permanent xerophytic characters. All true xerophytes are 

 evergreen e.g., yew, heath, pine, ling, box, laurel, many 

 succulents, etc. Plants which assume xerophytic char- 

 acters only at the approach of winter are tropophytes 

 (seep. 57). 



