428 Field Museum of Natural History — Botany, Vol. II 



dispersion, is evidently intended to perpetuate the species only in the 

 habitat of the parent plant. 



5. Chamaesyce buxifolia (Lam.) Small. 



This common seaside Euphorbia is found, on the booby rest-strands 

 only, on all the islets. It does not associate (except on Pajaros) with 

 the aquavectent Cakile nor with other aquavectent species. 



The seed coat of this species swells greatly and becomes mucilag- 

 inous when moist. This causes the seeds to adhere to the wet webs of 

 swimming birds when resting along the shore. Thus the plants are 

 widely distributed on the island strands of the Gulf of Mexico region. 



6. Tribulus alacranensis MiUsp. Field Mus. Bot. 2:54. 



This new species differs from its probable parent, T. maximus of the 

 mainland, in its long, heavy, tangled and jointed Ugneous stems and 

 branches; its mass growth; its larger flowers and smaller fruits; its 

 4-6-spined carpids; and in its leaves being densely woolly on both sur- 

 faces. The original habitat* of this species is apparently the Sporo- 

 bolus colony of Allison Island, where the boobies construct their nests 

 wholly of its stems and branches. 



The dispersional character of the species is avevectent, through the 

 sharp-spined fruits clinging to the webbed feet of swimming birds. 



7. Tournefortia gnaphalodes R. Br. Prod. 496. 



This common sea shore shrub of the Antillean region has found a 

 place of growth amongst the Siuiana shrubs at the southern extension 

 of the fringe on Perez. One shrub only was just appearing on the south 

 shore of Pajaros. The specimens appear to differ in no respect from 

 those of the mainland. 



As with Stiriana the fruiting branch tips of this species break off 

 dtuing high winds and are often blown into the sea, whence they are 

 drifted to new places of growth. The species is, again like Siuiana 

 bodily aquavectent in dispersional characters. It is very seldom, if 

 ever, found on the shores of bays or on shores guarded by partly dry 

 reefs. 



8. Conocarpus erectus Linn. Sp. PI. 147. 



A few clumps of this shrub have established themselves amongst 

 the Suriana bushes at the south end of the fringe on Perez Island, where 

 the shore being free on the weather edge of the reef, there has accumulated 

 quite a congregation of wave carried species. 



Two of the largest trunks of this species showed, by the armual 

 rings, a growth of 19 years. 



*As Utowana Cay was not explored this statement is open to doubt. 



