Vol. I] STEWART— BOTANY OF THE GALAPAGOS ISLANDS 231 



squalls on Charles Island during the months of August and 

 October, but none occurred on this island at the various occa- 

 sions we visited it, one of which was in the early part of 

 October. A thunder storm occurred around the top of Nar- 

 borough Island on March 21st, being the only one seen during 

 the entire year we spent among the islands. 



The effect of wind on the growth of vegetation is well shown 

 on the upper parts of Charles Island, where there are several 

 old tufa craters that rise from 500 to 800 ft. above the sur- 

 rounding table land in the interior of the island. The northern 

 sides of most of these are covered with a heavy growth of lime 

 and lemon trees, on the branches of which there are mosses 

 and other epiphytic plants. The southern and southeastern 

 sides of these craters, on the other hand, have only low peren- 

 nial herbs and bushes on them above 1350 ft., and only scat- 

 tered trees for two or three hundred feet below this elevation. 

 The change in the character of the vegetation is so abrupt in 

 these places that the two extremes often occur within a few 

 feet of each other. A somewhat similar but less pronounced 

 condition of affairs is found on the upper part of Chatham 

 Island, where the highest peak is covered on the leeward side 

 with a thick growth of Lycopodium clavatum and ferns. 

 Many of these are absent on the windward side, and those 

 species that do persist are only a few inches in height when 

 exposed directly to the action of the wind. Many species of 

 lichens are found growing on the rocks and twigs on this side 

 which are absent on the other. The trees of Bursera graveolens 

 lean in a northwesterly direction when they are exposed to the 

 wind, and their branches are often so bent and twisted as to 

 give the trees much flattened' crowns. 



Soil 

 The substratum for the most part consists of basaltic lava, 

 lava cinders, tufa, ashes, pumice, products derived from the 

 disintegration of these, sand, or vegetable mold. There are 

 many places in the dry regions where the lava flows are com- 

 paratively recent and there is practically no soil at all. Such 

 vegetation as is found there grows entirely from the crevices 

 in the lava. Basaltic lava or lava approaching basalt m char- 

 acter seems to form the best substratum for plants, as the 



January 16, 1911. 



