PIPER FLORA OF THE STATE OF WASHINGTON. 33 



THE ZONAL DISTRIBUTION OF WASHINGTON PLANTS. 



That there are physical causes which profoundly influence the dis- 

 tribution of plants no one who has crossed the State of Washington 

 from east to west can for a moment question. The contrast between 

 the treeless bunchgrass prairies and sage plains of eastern Washing- 

 ton and the luxuriant coniferous forests of western Washington is 

 too striking to overlook. In this particular case the principal factor 

 is one of humidity, the Puget Sound region possessing a notably 

 moist climate, while that of the Columbia Basin is markedly arid. 



A similar change of vegetation may be witnessed in the ascent of 

 any of our higher mountain peaks. As elevation increases the famil- 

 iar lowland plants disappear and different ones present themselves, 

 AAhich in turn give way at high altitudes to still others. The mc-l 

 marked of these changes is that where the timber ceases and the 

 alpine meadows present their charming carpet of flowers. Here the 

 changes are due manifestly not to differences in humidity, but to les- 

 sened temperature, a conclusion emphasized by the fact that mi-ny of 

 these alpine plants are the identical species which occur in arctic 

 regions. 



Heat and moisture are undoubtedly the principal physical factors 

 ui)on which the distribution of plants depend. A third factor may 

 l>e important, even determinative, namely, the character of thft soil, 

 Ijut this is much less potent than the two above named. In addition 

 to these physical factors only one other need be considered, the bio- 

 logical factor of ancestry. In general, plants inhabit the regions 

 where their ancestors thrived. This factor is usually continental in 

 its scope; thus cacti and yuccas are confined to America; eucalypti 

 to Australia, and lilacs to Asia. But in a similar way this same fac- 

 tor operates over small areas, and it is the principal cause why tlie 

 Pacific coast flora as a whole is different from that of the Atlantic. 

 The existing plants are different because their immediate ancestors 

 were, whatever factors may have determined that. 



It is not to be understood from this that all the plants which for- 

 merly flourished in Washington hare left descendants there. In Ter- 

 tiary times such plants as palms, cinnamon trees, and sequoias gi-ew 

 in Washington. Some of these require tropical or subtropical condi- 

 tions of heat; others, as the sequoias, probably have given way in 

 competition with more aggressive species. Nevertheless the bi'oad 

 statement remains true that the present vegetation of tu(i region owes 

 its character in large part to similar ancestors. Th3 conditions 

 which make the Pacific coast the home of many peculiar genera and 

 species are ancestral. The heat and moisture requirements of these 

 plants are duplicated in other portions of the. earth, where, however, 

 totally different congeries of species occur. / 

 29418—06 M 3 



