Key to the Genera 



I. Leaves needle-like or scale-like 

 1. Leaves alternate or fascicled 

 a. Leaves all alternate and scattered 



(1) Trees with fragrant leaves 



(a) Leaves square, inserted on prominent bases Picea : spruce 25 



(b) Leaves flat 



x. Leaves bright green, bract scales trifid, 



projecting Pseudotsuga; Douglas spruce 23 



y. Leaves dark and glossy above, whitish be- 

 low, bract scales rounded, included 

 (x) Cones large (5-10 cm.), erect, scales 



deciduous Abies: fir 20 



(y) Cones small (1.2-3cm.), pendent, 



scales persistent Tsuga : hemlock 24 



(2) Shrubs, leaves not fragrant 



(a) Leaves minute and scale-like, less than 2 mm. 



long Tamarix : tamarisk 52 



(b) Leaves more than 2 mm. long, not scale-like 



x. Leaves turned in at the edge, blunt Empetrum : crowberry 105 



y. Leaves flat, sharp-pointed Taxus : yew 41 



b. Leaves or some of them fascicled 



(1) Leaves deciduous, many in each fascicle Larix: larch ' 28 



(2) Leaves persistent, 2-5 in each fascicle Pinus : pine 30 

 2. Leaves opposite or in whorls of three 



a. Leaves in whorls of three, needle-like, jointed at 



base Juniper us : juniper 40 



b. Leaves opposite, scale-like, appressed on mature 



branches 



(1) Twigs flattened; pairs of leaves of two kinds 



alternating Thuja : arbor vitae 35 



(2) Twigs not flattened ; leaves all alike on the 



same twigs Sabina : red cedar 38 



II. Leaves not needle-like or scale-like 

 1. Trees 



a. Leaves simple 



(1) Leaves not lobed 



(a) Leaves ovate or round, broad 

 x. Leaves entire, mostly very large 



(x) Leaves opposite, mostly cordate at base Catalpa: catalpa 116 



(y) Leaves alternate, rounded or narrowed 



at base Magnolia : magnolia 45 



y. Leaves serrate to dentate 



(x) Twigs very tough; fruit round, per- 

 sistent, stalk winged Tilia : basswood 70 

 (y) Twigs (brittle ; fruit disappearing in 



spring Popuhts : cottonwood 53 



