THE LEAF. 65 



The leaflets of compound leaves of Flowering plants are ordinarily called 

 pinnoe, and their subdivisions lobes ; but in the Ferns, where the leaves 

 are highly compound, and the segments somewhat variable in the degree 

 of confluence, the primary divisions of the leaf are called pinnee, the 

 secondary pinnules, and the tertiary lobes or segments. In highly com- 

 pound leaves, the ramification of the petiole and subdivision of the lami- 

 nar structure become less complex toward the apex. 



Texture. The varieties of texture of ordinary leaves depend 

 chiefly upon their anatomical condition ; but it is requisite to 

 notice here several terms, such as membranous, leathery (or coriace- 

 ous), succulent, &c., used in Descriptive Botany, but which scarcely 

 require explanation. In aquatic plants the leaves are usually of 

 slighter texture : when they Jtoat on water (natant leaves) the forms 

 and general external characters are not much modified ; but when 

 they grow wholly under water (submerged leaves), they are not only 

 more delicate, but are sometimes cut up into fine filiform segments, 

 as in lianunculus aquatilis. 



Duration. The duration is different in different plants. Those 

 which are unfolded in spring and fall off in autumn are called 

 deciduous. What are called evergreen leaves vary in duration : thus 

 in ordinary evergreens, such as Ivy, Cherry-laurel (Primus Lauro- 

 cerasus), &c., the leaves remain through the winter and fall off 

 only when the new ones are becoming developed in the spring ; 

 while in many Conifers, as in species of Pinus, Araucaria, &c., the 

 leaves remain attached for many years. 



The anatomical structure of leaves exhibits many interesting modifica- 

 tions, related in some degree to the media and climates in which plants 

 grow. These will be more particularly explained in another place. 



Surfaces. The surfaces of leaves, like those of herbaceous 

 stems, exhibit a variety of conditions dependent on the character 

 of the epidermal layer. 



Glabrescent is used to signify that a surface, hairy when young, becomes 

 smooth when the leaf is mature, by the hairs falling off. Some 

 smooth surfaces are shining ; and this is very often the case with the 

 upper surface of evergreen leaves. Hairy surfaces are differently denomi- 

 nated, according to the character of the hairs and their mode of occur- 

 rence. Thus a pilose surface is covered with scattered soft and small 

 hairs, a hirsute with scattered long hairs, a hispid with scattered stiff 

 hairs; while a pubescent surface is covered closely with short soft hairs, 

 a villous closely with longish weak hairs ; and when the hairs are curled 

 and interwoven, the terms silky (sericeus), woolly (lanatus), felted (torncn- 

 tosus), Qijloccose, are applied according to the coarseness of the hairs and 

 the thickness of the coat they form. 



What may be called the natural smoothness of surfaces may be 

 interfered with by other irregularities analogous in their nature to hairs. 

 Slight, almost invisible rigid projections render the surface scabrous: 



