INTRODUCTION 

 NATURAL RELATIONSHIP OF TREES 



Trees, with the exception of tropical tree ferns, belong to one 

 of two major plant groups: the forms with naked seeds com- 

 monly subtended by a scale being known as Gymnosperms, 

 and the forms which contain an ovary which encloses the ovules 

 being known as Angiosperms. 



Two of the four orders of Gymnosperms contain trees which 

 grow in temperate climates, and by far the most important 

 of these are the cone-bearing trees such as the pines, spruces, 

 and cedars which are known as conifers (Coniferales) . The members 

 of this group have distinctive needle-like, linear, or scalelike leaves 

 and are often known as either evergreens (in spite of the fact 

 that some shed their needles annually) or softwoods (although 

 several have hard dense wood). Fifteen genera of conifers 

 containing 95 species are native to North America. These, 

 together with 8 exotic species, are treated in the first 1 1 8 pages 

 of this book. 



The other order of Gymnosperms present in tree form in the 

 region covered by this text is represented by a single surviv- 

 ing species, the ginkgo or maidenhair tree {Ginkgo biloba L.) 

 of China. This interesting tree is extensively planted 

 through all but the coldest parts of the United States and is 

 remarkably free from disease or inspects and very tolerant of 

 city smoke. Ginkgo forms a slender, spire-like tree (Fig. 1) 

 with fan-shaped, fern-like, deciduous leaves which are borne 

 either spirally on the twigs or at the ends of spur-like shoots. 

 The flowers are dioecious and of interest botanically because 

 of the free-swimming antherozoids. The fruit is orange-yellow 

 to green, about one inch in diameter, and resembles a plum 

 with its fleshy, ill-smelling, outer pulp enclosing a large, edible, 

 silvery pit. 



The Angiosperms likewise divide into two groups — the 

 Monocotyledons and the Dicotyledons. The first of these is 

 represented in this text by one palm, three palmettos, and seven 

 yuccas. These monocotyledonous plants are characterized by 

 parallel major veins in the leaf, flower parts in threes or sixes, and 

 fibrovascular bundles which are scattered through the stem. 



[xi] 



