KEY TO FAMILIES 



A\1ien the plant family of a tree is not known, 

 this key to the families of the trees of Puerto 

 Rico and the Virgin Islands aids identification. 

 Eighty-eight plant families are included, all 75 

 with native trees and also 13 others with common 

 introduced trees. Of these, 68 families are repre- 

 sented in this volume, 33 by a single species illus- 

 trated, however. Additional keys to the remain- 

 ing 218 of the 250 tree species ai-e in.serted in the 

 text under all 35 families with 2 or more species 

 illustrated. However, the related species com- 

 pai-ed briefly and various genera of less common 

 trees not mentioned in the text are not keyed. 



Some genera have been included in the key to 

 families also. If a plant family has only 1 or 2 

 genera of native trees, these generic names are 

 cited after the family name. Also, several genera 

 differing slightly from the main characters of 

 their families have been inserted separately. 

 However, this artificial key may not provide for 

 a few odd genera and extreme variations. Intro- 

 duced or exotic genera and families are indicated 

 by an asterisk ( * ) . 



Keys are outlines or shortcuts for identifying 

 trees or specimens by the process of elimination. 

 Thus, trees are divided into two groups according 

 to one or more pairs of contrasting characters. 

 Each group is divided successively into two 

 smaller groups until the name is reached. The 

 name of a particular specimen is found through 

 selection, one by one, of the group which fits and 

 by elimination of the others. 

 * In these indented keys, paired groups are desig- 

 nated by the same letter, single and double, be- 

 ginning" with "A" and "AA" at the left of the 

 page and are equally indented by steps. The page 

 number refers to the descriptive text, the be- 

 ginning of the family or genus listed. The 20 

 small families without page numbers are not men- 

 tioned further or represented among the common 

 trees described in this volume. 



An unpublished card key to families of West 

 Indian trees by the senior author has served as the 

 basis for this one. Nontechnical charactei-s and 

 those readily observed have been emphasized. 

 The descriptive terms are defined under the topic 

 "Explanation of Botanical Tenns" (p. 9). 



One character used in the key, presence or ab- 

 sence of stipules (one or two scales at the base 

 of a leaf), may be difficult to determine because 

 the stipules sometimes are minute or shed early. 

 Stipules can be examined best in the bud and im- 

 mature leaves near the stem tip. Upon shedding, 

 the stipules leave a scar, which also may be minute. 



Vegetative characters, especially those of leaves, 

 are placed first in the key. Some plant families of 

 trees can be recognized or identified by certahi 

 combinations of vegetative character alone. How- 

 ever, many kinds of tropical trees have foliage of 

 similar appearance. 



For positive identification of many families, the 

 reproductive characters of flowers, fruits, and 

 seeds are needed. Even when these are lacking, 

 old fruits may be located on dead branches or on 

 the ground, and one tree may be found flowering 

 out of season. Some sterile specimens, those lack- 

 ing flowei-s or fruits, can be identified to family 

 by the vegetative characters beginning the key. 

 If not, the key will eliminate many families. Then 

 identification can be continued by consulting the 

 text and drawings for the remaining families. 



The key to families is divided into four parts ac- 

 cording to the arrangement or position of the 

 leaves and the number of blades. Parts 1 and 2 

 are for trees w-th alternate leaves, that is, attached 

 smgly or 1 at a point on a twig (node). Parts 

 3 and 4 are for trees with opposite leaves, that is, 

 paired or 2 at a node, and also those with whorled 

 leaves, 3 or more at a node. Parts 1 and 3 are for 

 trees with simple leaves, with 1 blade, and Parts 



2 and 4, trees with compound leaves, divided into 



3 or more blades ( rarely only 2) . 



The first step in using this key to families is to 

 place the unknown tree or speciment in one of the 

 four groups listed below. Then continue the key 

 under the part or group on the page cited. 



Part 1. Leaves alternate, simple (p. 22). 

 Part 2. Leaves alternate, compound (p. 25). 

 Part 3. Leaves opposite, simple (p. 26). 

 Part 4. Leaves opposite, compound (p. 27). 



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