64 



PROPAGATION OF PLANTS. 



Fig. 23. LEAF OP CUT-LEAVED BIRCH. 



The general form of the leaf depends mainly upon the 

 disposition of the principal veins and branches of the 

 tissues of which the skeleton of the leaf is com- 

 posed. W^en these tis- 

 sues run parallel, and 

 are composed of a single 

 or several thread-like 

 fibres, the leaves will 

 assume a similar ,|orm, 

 as seen in the linear- 

 shaped leaves of the 

 Pine (figure 19, Jersey 

 Pine, P. mops). 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. Bjut in the simple 

 membranous leaf,~1ike that of 

 the common garden .Ldlaa, 

 figure 21, the central stem of 

 the leaf, for about one-third 

 of its length, tj^jyi what is 

 called a petiole or k-ai-stalk ; 

 then throws out branches, all 

 remaining united by the thin 

 membrane or parenchyma, 

 which fill the interspaces. J 

 of the Beech, figure 22, the secondary veins branch ofi 

 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 a saw. But in 



Fig. 24. LEAF OF BUCKEYE. 



some leaves, like those 



