Plant Invasion on Lava Flows. 23 



est, and consequently the densest forests on these islands are com- 

 posed of this species; but the prevailing tree in the upper forests 

 of the middle zone on the lee side of Hawaii is koa, while ohia is 

 apparently not the final type for this section. 



In the region below the wet forest the pahoehoe flows are 

 occupied almost solely by the naturalized flora, Psidium guajava^ 

 Lantana camara and introduced weeds being very conspicuous. 

 The long lines of native vegetation, consisting mainly of ohia, 

 which cross the Government road in main- places throughout 

 Koua, and the rich native flora at Puuwaawaa are on old aa flows. 



The region below the Government road and the shore line has 

 not been sufficiently investigated for discussion in this paper. 



In summing up the process of plant invasion on a lava flow 

 on the lee side of Hawaii the following sequence might be obtained: 



1 . Appearance of lower erytogams, eventually becoming con- 

 spicuous on the aa. 



2. Appearance of Polypodium pellucidum (folded form), Sad- 

 It ria cyatheoides and Metrosideros polymorpha , first on the pahoehoe, 

 and at a much later date on the aa. 



3. Gradual development of the typical floral aspects of the 

 immediate vicinity, if in the central region an ohia forest. 



4. Establishment of the final native vegetation, if in the cen- 

 tral region a koa forest. 



5. A later stage may be the encroachment of the naturalized 

 flora, due to a change of conditions brought about through human 

 agency. 



