146 SWITCH PLANTS, ETC. [CH. 



As a further illustration of the difficulties of explaining 

 the teleology of these adaptations it may be pointed out 

 that rolled leaves are characteristic of many plants which, 

 apart from periodical exposures for long periods to intense 

 isolation and drought, are liable to be heavily wetted at 

 intervals by mist, dew and mountain rains; and the sug- 

 gestion has been thrown out that the stomata thus escape 

 being blocked up by drops of water, owing to their position 

 in the grooves, and to the presence of hairs, wax, &c, on 

 the cuticle, so that the gas-interchanges are not interfered 

 with, even though the leaves of these mountain and 

 moor plants may be dripping with moisture in the early 

 mornings of days which will be hot and dry later. 



Yet another suggestion has been made regarding 

 inrolled leaves, namely that the air imprisoned in the 

 cavity by the inrolled margins acts as a protection against 

 the excessive chilling to which these plants are exposed. 



Another structural type of leaf, in part also character- 

 istic of plants exposed to the conditions referred to above, 

 is the cylindrical or terete leaf of Rushes, Hakea, Onion, 

 Isoetes, Subularia, &c. ; such are hardly distinguishable 

 from the likewise cylindrical stems of similar and of many 

 leafless plants Switch plants e.g. species of Scirpus, 

 Carex, Juncus, Equisetuni, Spartium, &c. 



In the leaves referred to the structure is radial, the 

 green tissues and the epidermis with its stomata being 

 distributed radially around the centre, and presenting no 

 upper and lower sides as in the typical bi-facial type of 

 ordinary leaves. 



It is not evident that adaptation to any one factor of 

 the environment can be regarded as probable in these 

 cases ; because, although such characters as the erect 

 position, depression of the few stomata in grooves, re- 

 duction of the transpiring surfaces by the assumption of 



