April, 1916. Alacran Reef — Millspaugh 431 



C. echinatus, glabrous in C. insiilaris); and in the shape and size as 

 well as the nervation of the first glume. 



The plant is dispersionally avevectent, the burs attaching themselves 

 by their sharp, barbed bristles to the webbed feet of swimming birds 

 during their walks along the strand. 



15. Philoxerus vermicularis (L.) R. Br.* 



This species, common on the shores of the lagoon at Progreso and 

 Silam on the Yucatan mainland, is found on the Alacrans only on a 

 bare spot in the Sesuvium colony of Perez and Allison Islands. 



From its position on the islands I am strongly inclined to judge its 

 dispersional character to be avevectent through the agency of small 

 migratory birds. 



16. Boerhaavia repens Linn. Sp. PI. 3.! 



This species was found only on one of the bare spots in the Sesuvium 

 carpet of Perez Island, where it nearly covered the small area in which it 

 had become implanted. It occurs also on the borders of the lagoon at 

 Progreso on the main but has not been gathered at any other part of 

 the Yucatan region. 



In regard to the Alacran Reef the dispersional character of this 

 species is avevectent, the viscous-glandular fruits attach themselves to 

 the feet and feathers of migratory birds. 



17. Opuntia Tuna (L.) Mill. Diet. ed. VIII :3. 



Two low, spreading cltimps of this cactus were observed growing from 

 the interstices in the cairn of old coral heads thrown up in the center of 

 the north half of Perez Island. The plants were vigorous and had 

 apparently well adapted themselves to their environment. 



Their location plainly indicates that, in this instance, the dispersional 

 character continues to be avevectent. That some migratory bird, 

 resting from flight upon the cairn, deposited the seed between the 

 topmost coral heads is evident. 



18. Scaevola Lobelia Murr. Linn. Syst. ed. XIII :i 78. 



One individual only of this seaside species was seen upon the islets 

 of the reef. This was growing directly under the left arm of the little 

 wooden cross erected at the head of the grave on the north end of Perez 

 Island. I removed a portion of this plant for herbarium purposes: 

 this may possibly affect its continuance. 



The position in which the plant grew leaves no doubt as to the 

 avevectent character of its dispersion. 



*Lithophila vermiculata Uline in Field. Mus. Bot. 2:39. 



tDr. Heimerl considers our Alacran plant to be nothing but a form transitional 

 between the B. viscosum and B. hirsuta of Lagasca. 



