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



tion should prove particularly interesting, we were deeply disappointed 

 that the risk was too great to allow of the yacht being brought near 

 enough to grant us a landing. 



THE PLANT SPECIES 



1. Sesuvium portulacastrum (L.) Linn. Syst. ed. X, 1058. 



This is by far the prevalent species of the cays and here forms the most 

 extensive and succulent masses I have ever seen. It clothes the easterly 

 areas of all the islets with a dense carpet about eight inches thick. Only 

 a few individuals were in flower at the time of our visit. As the stems 

 of this plant are fragile; broken pieces long-lived; and as such pieces 

 put forth strong and viril rootlets in water, I am inclined to place its 

 dispersional character as bodily aquavectent. It is possible also that 

 ripe fruits may be transported in like manner. Its characters are strong- 

 ly against avevectence. 



2. Sporobolus virginicus (L.) Kunth. Rev. Gram. 1:67. 



The second species in extent on the cays. This grass appears to 

 spread only where some influence has checked the growth of the Sesuv- 

 ium, as for instance the tramping about of the birds or the digging of 

 water-holes and the constructive efforts of the men who built the beacon 

 structure on the south end of Perez. This is so pronounced a fact that 

 a glance at the distinguishing colors on the maps shows by the yellow, 

 for Sporobolus, the location of the three water-holes and the area of 

 the beacon-site; while the yellow on the maps of the other cays demarks 

 perfectly the booby nesting areas — always, for some reason westerly 

 on the islets. Note, by the same token, that the booby sections are 

 always where the land slopes gently to spit-like extremities.* The booby 

 never nests near the weather, or abrupt, shore. 



From its habitat it is evident that Sporobolus is aquavectent; it has 

 however only been found, on the Yucatan mainland, in protected situa- 

 tions near lagoons, whence, if its Alacran soiu*ce is the mainland, its 

 spikes must first have been blown to the sea before its real transportation 

 began. 



3. Suriana maritima L. Sp. PL 284. 



A fringe of low shrubs of this species lines the top of the ''bank" 

 of the western or weather shore of Perez Island. The growth is pure in 

 the northern two-thirds of the fringe, the southern third has an inter- 

 mixture of a few clumps of Toumefortia and, further south, of Conocar- 

 pus. 



* In order that the fledglings may get to the water and back to the nest? 



