MODIFICATION OF FORM 211 



on the inner surface. The rolling may be permanent as in Empetrum and 

 members of the Heath family, or may only appear during dry spells, as in 

 various Grasses (p. 188). In some Xerophytes the leaves occupy a vertical 

 rather than a horizontal plane, due to a bending of the petiole, as in certain 

 species of Eucalyptus. The incidence of the sun's rays on the leaf is thereby 

 much reduced. A similar feature is commonly found in those plants in which 

 the leaves are replaced by flattened stems or petioles. Butcher's Broom 

 (Ruscus) and certain Acacias are examples of these (Fig. 259, p. 344). 



Recent experimental work has revealed that when water is available some 

 Xerophytes show a surprisingly extensive loss of water by transpiration, 

 perhaps related to the need for rapid photosynthesis and growth during 

 the infrequent wet spells. They cannot as a class be said to show low 

 transpiration at all times. During dry periods, however, the possession 

 of heavy cuticle, of hairs and of protected stomata which may remain 

 closed, doubtless tends to reduce transpiration. A further property of 

 Xerophytes is the ability of their protoplasm to withstand desiccation to an 

 unusual extent. 



Modifications like those shown by xerophytes are seen also in other 

 plants where water-supply is for other reasons difficult, as it is in those 

 which live attached to the branches or trunks of other plants (Epi- 

 phytes). Since they have no direct access to the soil, they must receive 

 and store the water from rainfall, or condense it from a moist atmo- 

 sphere. This is the condition of many tropical Orchids and Bromeliads. 

 In the latter a special surface-protection is afforded by scurfy peltate 

 hairs, while others serve for absorption of water. 



Again, in Arctic and Alpine plants many xerophytic characters are 

 presented, such as deep rooting, leaf-reduction, succulence, waxy 

 surface, or hairy coverings. These are probably related to the 

 condition of physiological drought caused by the prevailing low 

 temperature of the soil, which checks the activity of root-absorption : 

 while the shoot, in clear weather and in a wind, may be exposed 

 to conditions which would stimulate transpiration to a dangerous 

 degree. The same applies to the temporary reduction of leaf-area 

 of deciduous trees in winter (see p. 194)- 



Halophytes living on the sea-shore, or in salt-marshes, also show 

 characters similar to xerophytes, such as reduced leaves and succulence 

 (Salicomia), and development of spines (Salsola). To explain this it 

 has been assumed that halophytes experience physiological drought 

 because of the high osmotic pressure of the saline soil solution. Experi- 

 mental investigation has not supported this theory, for halophytes 

 transpire freely and appear to have no difficulty in securing water, 

 while despite their succulence they cannot withstand drought. Their 

 resemblance to xerophytes may be an apparent one only. 



