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 

 tb. ' treeless bunchgrass prairies and sage plains of eastern Washing- 

 ton and the luxuriant coniferous forests of western Washington is 

 loo 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 Avitnessed in the ascent of 

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

 iar lowland })lants disappear and different ones present themselves, 

 >\hich in turn give way at high altitudes to still others. The me.-t 

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

 alpine meadows present their charming car})et of floAvers. Here the 

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

 sened temperature, a conclusion emphasized by the fact that mrny of 

 these jilpine j^lants are the identical species which occur in arctic 

 regions. 



Heat and moisture are undoul)tedly the principal physical factors 

 r,])on which the distribution of plants depend. A third factor may 

 be im})ortant, even determinative, namely, the character of the soil, 

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

 to these i)hysical factors only one other need be considered, the bio- 

 logical factor of CDicc-stry. In general, plants inhabit the regions 

 where their ancestors thrived. This factor is usually continental in 

 its scoi^e; thus cacti and yuccas are confined to America; eucalypti 

 to Australia, and lilacs to Asia. But in a similar wa}^ this same fac- 

 tor oi)erates over small areas, and it is the principal cause why the 

 l*acific coast flora as a whole is different from that of the xVtlantic. 

 The existing plants are different because their immediate ancestors 

 were, whatever factors may have determined that. 



It is not to l)e 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 se([uoias grew 

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

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

 comiK'tition with more aggressive species. Nevertheless the broad 

 statement remains true that the present vegetation of XXa^ region owes 

 its character in large part to similar ancestors. Thi 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 



