﻿Trees of New York State 29 



species, but even within one and the same species and on the 

 same plant. Variation in the size, shape, margin, etc., of foliage 

 leaves is of the utmost importance in tree identification. 



Leaf Classification 



A. Leaves classified as to Position (Phyllotaxy) 



1. Alternate — one inserted at a node. 



2. Opposite — two at a node, inserted on opposite sides 



of the stem. 



3. Verticillate or whorled — three or more at a node. 

 Alternate leaves are arranged in ascending left to right spirals 



on the stem and according to a definite plan. The portion of the 

 spiral from one leaf to one directly above it is termed a cycle. 

 Spiral phyllotaxies are represented by the fractions y^, %, %, 

 etc., in which the numerator represents the number of turns and 

 the denominator the number of leaves in a given cycle. 



B. Leaves Classified as to Composition 



1. Simple, with undivided lamina. (Fig. 1.) 



2. Compound, lamina divided into leaflets. 



a. Pinnately compound, leaflets arranged along the 



two sides of the rachis. 



(1) Odd pinnately compound, with an odd 



number of leaflets.* (Fig. 2.) 



(2) Even pinnately compound, with an even 



number of leaflets. 



(3) Pinnately decompound, twice pinnately 



compound. (Fig. 3.) 



b. Palmately compound, leaflets radiating from the 



ends of the petiole. (Fig. 4.) 

 (1) Ternately decompound, palmately decom- 

 pound in threes. 

 C. Leaves Classified as to Form 



1. Acicular, long and very slender; needle-like. (Fig. 



5.) 



2. Subulate, awl-shaped; short, sharp-pointed, broad- 



ened at the base. (Fig. 6.) 



3. Linear, narrow, several times longer than wide, and 



with sides approximately parallel. (Fig. 7.) 



4. Oblong, longer than broad, and with approximately 



parallel sides. (Fig. 8.) 



Lciitlt'ts may be horne (ippositc ov iiUcriiatf on the rachis in (1) and (2). 



