110 BOTANY PART i 



not extend half-way to the middle of the leaf-blade, when they reach half-way as 

 CLEFT (Fig. 135 sb), and when still deeper as PARTITE (Fig. 137 I). The lamina 

 is PALMATE (Fig. 137) or PINNATE (Fig. 136, 1-5), according to whether the 

 divisions are directed towards the base of the leaf-blade or towards the midrib. 

 Only when the separate divisions are so independent that they appear as distinct 

 leaflets borne, on a common petiole or on the original midrib is the leaf spoken of 

 as COMPOUND (Fig. 136, 1-5) ; in all other cases it is termed SIMPLE. 



The leaflets of a compound leaf may be so segmented during their development 

 as to resemble the main leaf, and in this way a leaf may be doubly or triply com- 

 pound or more highly segmented. Simply pinnate or bi-pinnate leaves (Fig. 136) 

 bearing leaflets on the two sides of the rachis of the first or second order are of 

 frequent occurrence. The leaflets of a compound leaf may be entire or more or less 

 deeply incised. They may be inserted directly on the rachis or be stalked, and in 

 some cases, e.g. Phaseolus (Fig. 132 fg], Robinia, Mimosa, be provided with 

 swollen pulvini at their bases. 



In laminae, which become more or less branched during their development, the 

 lateral divisions usually arise in basipetal order, i.e. proceeding from the tip 

 towards the base, but the opposite (acropetal) succession or a combination of the 

 two are also met with. The divisions of palmate and pinnate leaves of the Palms 

 arise by a relatively late process of splitting within the originally entire, enlarging 

 lamina. The direction of the dividing lines is determined by the folding of the 

 young leaf-blade ( 61 ). 



Sessile leaves usually clasp the stem by a broad base. Where, as in the case 

 of the Poppy (Papaver somniferum), the leaf-base surrounds the stem, the leaves 

 are described as AMPLEXICAUL ; if, as in species of Bupleurum, it completely 

 surrounds the stem, the term PERFOLIATE is used. If the bases of two opposite 

 leaves are united, as in the Honeysuckle (Lonicera caprifolium), they are 

 said to be CONNATE. Where the blade of the leaf continues downwards along 

 the stem, as in the winged stems of the common Mullein ( Verbascum thapsiforme), 

 the leaves are distinguished as DECURRENT. 



The leaf-blade is traversed by green nerves or veins which form 

 a branched net- work. The thicker ribs project more or less from the 

 surface on the lower side of the leaf, the upper surface often showing 

 corresponding grooves. The finer veins become visible when the leaf- 

 blade is viewed by transmitted light. Frequently the nerve in the 

 middle line of the lamina is more strongly developed and is then termed 

 the midrib ; in other cases several equally developed main nerves are 

 present (Fig. 122). Lateral nerves spring from the one or more main 

 nerves (Fig. 126). 



The course of the nerves determines what is known as the VENATION of the 

 leaf. The leaves of most Coniferae are UNI-NERVED. In leaves with more 

 numerous veins, the DICHOTOMOUS VENATION must be distinguished as a special 

 type which is characteristic of many Ferns and is also found in Ginkgo biloba ; 

 there is no midrib present in this case. Most other leaves can be distinguished 

 according to their venation as PARALLEL VEINED or NETTED VEINED. In parallel 

 venation the veins or nerves run either approximately parallel with each other or 

 in curves, converging at the base and apex of the leaf (Fig. 133 s) ; in netted 

 veined leaves (Fig. 126) the veins branch off from one another, and gradually 

 decrease in size until they form a fine anastomosing network. In leaves with 



