950 A TEXTBOOK OF THEORETICAL BOTANY 



with descriptive names. At the present day this kind of study is chiefly of 

 interest to the systematist and students are not often introduced to the rather 

 bewildering assortment of terms employed. The study of systematics is, 

 however, regaining some of the ground which it lost during the post- 

 Darwinian era, for it has been realized that systematic relationships are an 

 essential part of a plant's being and are often valuable guides in problems of 

 physiology, ecology, and above all of genetics. 



We present, therefore, a simplified and illustrated classification of leaf 

 forms which covers the main types. If any student is inclined to cavil at 

 such a multiplicity of technical terms let him reflect what would be the 

 difficulty of describing these variations or indeed anv organic forms without 

 their aid. Botanical terminology has been evolved to lessen labour, not to 

 increase it, as some are apt to suppose. 



Leaves are classified, in the first place, as either simple or compound. 

 A simple leaf may have a divided or undivided lamina, but in the former 

 case the portions of the divided lamina are not wholly separated from each 

 other or from the midrib of the leaf. A compound leaf is composed of 

 separate leaflets, each of which has a separate and distinct base, attached to 

 a common leaf stalk, aiid each may have a small secondary leaf stalk of its 

 own. 



Either type of leaf may have a stalk or petiole, which carries the blade 

 or lamina. Alternatively the leaf may be sessile, that is, attached directly to 

 the stem at the base of the lamina, with no intervening petiole. When the 

 base of a sessile leaf surrounds the stem at the node, it is called amplexicaul. 

 When an amplexicaul leaf is extended beyond the stem on the far side of the 

 node, so that the stem appears to be growing through the leaf, it is called 

 perfoliate. i 



If the lamina of a sessile leaf is prolonged downwards from the point of 

 attachment at the node, so that it forms a membrane or frill, attached to the 

 stem, the leaf is said to be decurrent. When two sessile leaves stand 

 opposite each other at a node, and their bases are joined to each other round 

 the stem, they are called connate. 



The divisions of the lamina, whether in simple or in compound leaves, 

 may be arranged either in two series, on each side of a common axis, in 

 which case they are said to be pinnate ; or they may form a radiating series 

 like the fingers of a hand, in which case they are called palmate. In a 

 simple pinnate leaf the common axis is the midrib of the leaf, but in a 

 compound leaf it is a prolongation of the petiole called the rachis, as in the 

 compound leaves of Ferns. A pinnate leaf with an even number of paired 

 leaflets is said to be paripinnate, but if there is an odd terminal leaflet it is 

 called imparipinnate. 



If the margin of a leaf is smooth and without indentations it is said to be 

 entire, while the various degrees of indentation, with their names, are 

 illustrated in Fig. 937. A distinction is drawn between indentations which 

 reach to the midrib, thus dividing the lamina into separate portions, and the 

 less marked divisions which leave a continuous portion of the lamina along 



