INSULAR FLORAS. 301 



in the genus for its very small flowers, which are only about 

 three-quarters of an inch in diameter, and also for the small 

 number of petals ; but as the figure was made from dried 

 specimens, it may be inaccurate in some details. In the 

 same place it is mentioned that a species of Cereus was 

 common in the island, but was not found in flower. 



Darwin himself specially alludes (44) to the prominent 

 feature these Cactacese are in the landscape, and likewise 

 to the fact that they grow in the rough lava where there is 

 absolutely no other phanerogamic vegetation. He further 

 points out their importance as food for the gigantic tor- 

 toises and land lizards. They are also a source of water 

 during the severe droughts, which often parch the lower 

 zone. 



Subsequent travellers have dwelt upon the part the 

 Cactacese play in the biology of the island, and Andersson, 

 a botanist who visited the islands in 1852, states (45) that 

 he observed four or five species, but had time neither to 

 prepare specimens nor sketch the plants. 



My note on the subject in Nature came under Dr. 

 Baur's notice, and he forwarded me two photographs, one re- 

 presenting a fine example of an arboreous Opuntia of great 

 size, and the other a view embracing a number of large 

 Cerei, together with a transcript of his notes on the subject 

 in a paper (46) which I had not seen. He was struck by 

 the difference in the appearance of the Optmtice on the 

 different islands, and observed that the large Opuntia has 

 a different habit on nearly every island. Thus, on Barring- 

 ton, Indefatigable and South Albemarle, it develops a 

 very tall stem ; on Charles and Hood a relatively short 

 but thicker stem ; on Jervis a very short stem, branch- 

 ing from very near the ground, and on Tower Island 

 it forms no stem at all, and appears as a dwarf bush. 

 Dr. Baur attributes these modifications to the varying degrees 

 of humidity, the greatest development occurring in the driest 

 climate. In the lower region of South Albemarle, up to 

 about 500 feet, the Opuntia is very common, attaining a 

 large size, the largest being about twenty feet high, with a 

 trunk two feet in diameter. " In old trees the bark looks 



