FOREST FORMATIONS AND FOREST TREES 53 



2a. Trees without thorns or spines. (3) 



2b. Trees with thorns or spines. (10) 



3a. Trees with rough bark of bitter taste; leaves much longer than broad; flowers and 

 fruits in erect tat kins; seeds cottony (willows). Genus 7. Salix 



3b. Trees with white or light gray branches; flowers and fruits in drooping catkins; 

 seeds cottony (cottonwoods, poplars and aspen). Genus 6. Populus 



3c. Trees with conspicuous flowers and fleshy fruits (cherry, plum and June berry). 



(4) 



3d. Trees not falling under 3a, 3b or 3c. (5) 



4a. Leaves with margin finely toothed all around (plum and cherry). ^ 



Genus 15. Prunus 

 4b. Leaves with margin toothed only in the upper half (June berry). 



Genus 13. Amelanchier 



5a. Leaves with veining pinnate; a distinct mid-rib with lateral veins extending from 

 it. (6) 



5b. Leaves with palmate veining; the main veins extending from the base of the leaf- 

 blade like fingers from the palm of the hand (maples). Genus 1 7. Acer 



6a. Leaves elm-like; not symmetrical at base (hackberry). Genus n. Celtis 



6b. Leaves not elm-like; usually symmetrical at base. (7) 



7a. Leaves very small; only 15 to 25 mm. (J to 1 in.) long (mountain mahogany). 



Genus 12. Cercocarpus 

 7b. Leaves mostly 35 mm. (if in.) long, or longer. (8) 



8a. Leaves pinnately lobed or cleft; the lobes and sinuses rounded; fruit an acorn 

 (oaks). Genus 10. Quercus 



8b. Leaves merely toothed or slightly sharp-lobed; flowers and fruits in catkins (alders 

 and birches). (g) 



8c. Leaves finely -toothed or nearly entire; fruit berry-like, in small clusters (cascara 

 sagrada). Genus 19. Rhamnus 



8d. Leaves finely toothed or wavy margined; fruit a winged samara (ash). 



Genus 20. Fraxinus 



pa. Leaves rough and coarse; with both fine and coarse teeth (alder). Genus 9. Alnus 

 9b. Leaves rather smooth; bright green. Genus 8. Betula 



10a. Spines numerous, sharp pointed (thornapple). Genus 14. Crataegus 



10b. Spines few, scattered (plum). Genus 15 Prunus 



1 1 a. Trees with short thorns in pairs (locust). Genus 16. Robinia 



lib. Trees without thorns; leaflets more than four pairs (sumac). Genus 18. Rhus 

 lie. Trees without thorns; leaflets 3, 5 or 7 (box-elder). Genus 17. Acer 



DESCRIPTIONS AND KEYS TO SPECIES 



Genus 1. PINUS, Pine 



Evergreen trees and shrubs growing often in dry and rocky situations. 

 Foliage leaves needle-shaped growing usually in bundles of 2 to 5 



