XVlll 



North American Trees 



the plant. Thus, the scientific name of ponderosa pine is Pinus 

 ponderosa Laws. When a variety of a species is recognized, the 

 varietal name follows the specific name, so in the case of Arizona 

 pine, a southern variety of ponderosa pine, the scientific name 

 Pinus ponderosa var. arizonica (Engelm.) Shaw. When the 



is 



names of two authorities are given, one appearing in parentheses, 

 it indicates that the species was first described by the authority 



Fig. J— FLOWER STRUCTURE 



Perfect Flower Flowers of Pine 



1 -Peduncle. 2-Receptacle. 3-Sepal (Calyx). 4-Petal (Corolla). 

 5-Stamen; a- Anther, b-Filament. 6-Pistil; a-Stigma, b-Style, 

 c-Ovary, d-Ovule. 7-Staminate conelet. 8 and 9-Stanien, or pollen- 

 bearing scale, showing side and lower surfaces. 10-Pistillate conelet. 

 11 and 12-Pistillate scale showing inner and outer surfaces respec- 

 tively; a-Ovule, b-Bract. 



indicated in the parentheses, but in a diff'erent group of the same 

 rank, or in a different rank, and that the second author was the 

 one that had eff'ected the change. This is the case with the 

 western hemlock, Tsuga heterophylla (Raf.) Sarg. In this work 

 the nomenclature accepted by the United States Forest Serv- 

 ice (as set forth in the revised Check List of 1953) has been 

 followed. 



Habit. This refers to the general appearance of a tree, usually 

 as seen from a distance. The size; appearance and form of 

 the trunk; shape, density, and size of crown; the number, size, 

 and direction of growth of the branches, are all factors help- 

 ful in distinguishing trees. In the conifers there is typically 

 a trunk that extends to the tip of the tree without dividing 

 (excurrent), while most of the hardwoods have the trunk break- 

 ing up into several large branches (deliquescent). Both the 

 crown and branches may vary greatly in the same species, 



