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species. Three of these are not native to the Pacific Northwest: <br /> the Silver linden is Tilia tomentosa of European extraction; the <br /> American elm Ulmus americana comes no closer than the Rockies; and <br /> the American ash Fraxinus americana is, likewise, an eastern <br /> species. <br /> The Madrona Arbutus menziessii is essentially a large <br /> broadleaf evergreen shrub until it reaches a very ripe old age. <br /> The City's rule on vision clearance requires that each such tree be <br /> headed up to nine feet. It is impossible to conceive of any way in <br /> which madrona could be planted in the median planting island and <br /> still ensure pedestrian safety. <br /> The Oregon oak Ouercus garryana is a difficult thing to use <br /> within the restraints of the City's programs. There is no source <br /> of procurement, and the city has no nursery to grow its own for <br /> four or five years to the 1 1/2 inch caliper planting stage. Even <br /> if some were available and were planted, they would display almost <br /> no sprouts for two or three years while they grew the massive tap <br /> root system it requires, and for the next 25 years it would be a <br /> very dull scrub oak, a very unlovely thing. Eventually it does <br /> develop that sturdy, handsome round head that one admires in large <br /> open spaces, but it cannot pay any downtown rent in performance on <br /> the streets for the first twenty five years. It may get by in some <br /> laid -back residential area, but abutting property owners in <br /> downtown must expect a more sophisticated embellishment for the <br /> economic values involved there. <br /> The native dogwood Cornus nuttallii flourishes beautifully on <br />