THE LEAF 



two lateral leaflets, so as to form a rhachis, as in Figure 66, 

 the leaf is pinnately trifoliolate ; but 

 if all three appear to spring directly 

 from the top of the petiole, as in 

 Figure 67, it is palmate.. A good exam- 

 ple of a pinnately trifoliolate leaf, and 

 one which it is important to learn and 

 remember, is the poison ivy. 



47. Unity of Plan in Nature. 

 Notice how the same plan of structure 

 runs unchanged through all these 

 .poison ivy. variations. If an oak or a tansy leaf 

 were cut through to the midrib, we should have a pinnately 

 compound leaf, while a sweet gum or a maple cut in the 

 same way would give rise to a palmately compound one. 



48. The Branching of Leaves. Lobed and compound 

 leaves represent mere degrees of branching. Notice, how- 

 ever, that their mode of branching differs from that of 

 stems in having the branches all in the same plane, like 

 figures cut out of a single sheet of paper. This is what 

 we should expect in the case of expanded bodies whose 

 primary object is exposure to the light. 



49. What makes a Compound Leaf. Some botanists do 

 not regard a branched leaf as compound unless the leaflet* 

 are jointed to the common petiole so that they break and 

 fall away separately in autumn, like those of the ash, 

 horse-chestnut, china tree, etc. According to this defi- 



69. Leaf of common orange. 



7- Leaf of trifoliolate orange. 



