64 



PBOPAGATIOM- OF PLANTS. 



Fig. 23.— LEAP OF CCT-LEA-VBD BIBCH. 



The general form of tie leaf depends mainly upon the 



disposition of the principal Teins and branches of the 



woody tissues of which the skeleton of the leaf is com- 

 posed. When these tis- 

 !^^v"\^ . sues run parallel, and 



are composed of a single 

 or several thread-like 

 fibres, the leaves wiU 

 assume a similar form, 

 as seen in the linear- 

 shaped leaves of the 

 Pine (figure 19, Jersey 

 Pine, P inops). The 

 veins in these leaves 



starting singly from the stems, are each surrounded or 



incased in cellular matter. This single form of growth 



is common in many of the conifers ; the leaves, instead 



of having branching veins, grow in. a thread-like bundle 



or fascicle, as in the Larch, 



figure 20. But in the simple 



membranous leaf, like that of 



the common garden Lilac, 



figure 21, the central stem of 



the leaf, for about one-third 



of its length, forms what is 



called a petiole or leaf-stalk ; 



then throws out branches, all 



remaining united by the thin 



membrane or parenchyma, ^S- 2*— i^^ o^' b^ckete. 



which fill the interspaces.. In some leaves, like those 

 of the Beech, figure 33, the secondary veins branch off 

 at an ascending angle from the mid-rib, running almost 

 in a straight line to the outer edge of the leaf, forming a 

 saw-tooth-like notch where each terminates. The edges 

 of such leaves are said to be serrate, because beset with 

 teeth projecting forward like those of » saw. But in 



