no ADAPTATIONS. OECOLOGICAL CLASSIFICATION sect, hi 



stellate manner. Polytrichum can lay the marginal portion of the 

 leaf over the thin-walled assimilatory cells clothing the more central 

 portion.^ 



B. Permanent Reduction of Form of Leaf and Shoot. 



In very many xerophytes the transpiring organs, the foliage-leaves, 

 are extremely and unalterably reduced in size and surface, and there 

 result a number of speciahzed types of xerophytic shoots. The size 

 of the foliage-leaf, and of the foliaged shoot as a whole, shows a certain 

 dependence upon the amount of food-material and water available 

 to the plant at the time of its development. Lack of water may induce 

 nanism ; for instance, in dry sandy places many species are dwarfed ; 

 again, one and the same species may be small-leaved on dry soil, and 

 large-leaved on moist soil, as is the case with Urtica dioica, Viola canina, 

 Erodium cicutarium and many others ; a number of desert - plants, 

 including Zilla and Alhagi, produce at the commencement of the rainy 

 season large leaves, but later on much smaller ones or none at all. The 

 smallness of the leaf is a direct result of dryness.^ Lack of water has 

 apparently also contributed to the evolution of a series of definite fixed and 

 constant types, which are characterized by their relatively low assimilatory 

 power and consequent slowness of growth.^ These types are described 

 in the succeeding paragraphs. 



{a) Forms of Leaf. 



1. The pinoid or acicular leaf is met with in Coniferae, Proteaceae, 

 Ulex europaeus, and others. It is long, linear, pointed, and often has 

 a more or less radial structure. The relations of this leaf to transpira- 

 tion result from the fact that its surface in proportion to its volume 

 is much less than in the case of a fiat leaf, hence its evaporating surface 

 is relatively less. This is also true of the forms of leaves described in the 

 succeeding paragraphs. 



2. The ericoid leaf is a rolled leaf, in other words, its margins are curled 

 downwards, or upwards (much more rarely, as in Passerina) ; there 

 thus arises a furrow in which the stomata are secluded from movements 

 of the air.* Ericoid leaves are short and linear ; they occur in Erica, 

 Calluna, Cassiope tetragona and other Ericaceae, Epacridaceae, Myr- 

 taceae, Berberis empetrifolia from Chile, South-African Thymelaea- 

 ceae, Compositae, Rubiaceae, and among other families in species growing 

 on maquis, heaths, or other places where transpiration is strong. 



3. The cupressoid (lepidophyllous, lepidoid) leaf is broad and short, 

 appressed, apically directed, and sometimes decurrent ; it is met with 

 in many Cupressaceae, in some Scrophulariaceae (in Veronica thuyoides 

 and V. cupressoides, which are alpine in New Zealand), Santalaceaej 

 Tamaricaceae, Compositae, Umbelliferae (in Azorella at alpine altitudes 

 in South America, and in Antarctic lands).^ 



4. The setaceous ov filiform leaf occurs in very many grass-like Mono- 

 cotyledones ; it is usually furrowed or channelled on its upper face, and 



^ Kerner, 1887. '' Henslow, 1894; Scott-Elliot, 1905; Percy Groom, 1893. 



' See Chap. XCIX. " See p. 105. * Gobel, 1891 ; Lazniewski, 1896. 



