XX 



North American Trees 



a 



-iizd 



d— 



c- 



by means of which the species is perpetuated. Floral characters 

 are the most accurate means of identifying many trees (Fig. 

 3), although they are little used in the field because they bloom 

 for so short a period. Flowers vary greatly in form, structure, 

 and size. A complete flower (Fig. 3) is made up usually 

 of leaflike sepals (calyx), often brightly colored petals (corolla), 

 stamens (the male organs which bear the pollen in saclike 



anthers), and a pistil 

 (the female organ con- 

 sisting of a terminal 

 stigma which catches the 

 pollen, a style, and an 

 ovary). The ovary may 

 consist of one or more 

 compartments (cells) and 

 contain one to many ovules 

 which later mature into 

 seeds. 



If the ovary is inserted 

 on top of the other flower 

 parts it is superior, while 

 if it appears below the 

 origin of sepals, petals, and 

 stamens it is inferior. Flowers 

 may be perfect (contain 

 both stamens and pistil) 

 or imperfect (contain one 

 sex but not the other). 

 Plants having imperfect 

 flowers are termed either 

 monoecious (both sexes 

 present in diff'erent flowers 

 on the same plant) or 

 dioecious (each sex borne 

 on a diff'erent plant). When 

 the only functioning sex 

 organs in an imperfect flower are stamens the flower is termed 

 staminate, while one in which the pistil is the active organ is 

 pistillate. Plants which bear some perfect and some imperfect 

 flowers are termed polygamous. Flowers may appear singly or 

 in clusters (inflorescences, Fig. 4). In Gymnosperms the pistil 

 is replaced by a seed-bearing scale which does not enclose the 

 seed. 



m 



Fig. 5 — (Left) Winter Twig of 

 Black Walnut, (a) terminal bud; 

 (b) lateral bud; (c) leaf scar; (d) 

 bundle scars; (e) pith. 



(Right) Winter Twig of Red 

 Mulberry, (a) tip scar; (b) lateral 

 bud; (c) leaf scar; (d) stipule scars. 



