40 



THE LEAF 



68. — Poison ivy. 



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 

 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 leaflets 

 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. 



70. — Leaf of trifoliolate orange 



