Vol. I] STEWART— BOTANY OF THE GALAPAGOS ISLANDS 219 



Lianes occur in the following genera: Argyreia, Asclepias, 

 Boussingaultia, Canavalia, Cardiospermum, Cissampelos, 

 Cissus, Elaterium, Galactea, Ipomoea, Momordica, Mucuna, 

 PassiHora, Phaseolus, Rhynchosia, and Sicyos. Most of these 

 are herbaceous. 



Those plants which attain the size of trees are included in the 

 following genera : Acacia, Acnistus, Avicennia, Bursera, 

 Cereus, Conocarpus, Erythrina, Hibiscus, Hippomane, Opun- 

 tia, Piscidia, Pisonia, Psidium, Prosopis, Rhisophora, Sap- 

 indus, Scalesia, Solanum, and Zanthoxylum. More than one 

 half of these are confined to the regions below the moist, con- 

 trary to the general belief that the lower parts of the islands 

 support only a low and bushy vegetation outside of the arbor- 

 escent cacti. A few of the above attain sufficient size to be of 

 economic importance for lumber, among which the species of 

 Erythrina, Psidium, and Sapindus are the most important. 



The greater number of species of plants have small and 

 rather inconspicuous flowers, a fact that has been mentioned by 

 other travelers who have visited the islands. There are a few 

 plants, however, that possess rather showy flowers. Such are 

 comprised in the genera Argyreia, Cacabus, Cereus, Cordia, 

 Datura, Erythrina, Gossypium, Hibiscus, Ipomoea, Kallstroe- 

 mia, Miconia, Mvicuna, Nicotiana, Opuntia, Parkinsonia, Passi- 

 Uora, and Tribulus. Most of these genera include species of 

 wide distribution. By far the largest number of endemic spe- 

 cies have very small flowers, a fact that may be due to the 

 relatively small number of species of insects on the islands. 



Ecological Factors 



Water 



Great differences in the amount of precipitation are often 

 found within short distances on the Galapagos Islands, some- 

 times within a change of elevation of two or three hundred feet. 

 The lower parts of the islands adjacent to the shore are as a 

 rule very dry and only receive moisture in any considerable 

 quantities during about three months of the year, while the 

 middle and upper parts are quite moist most of the time. 

 Between the two extremes of moisture there are all sorts of 

 gradations. 



