Botanical Ecology of the Dry Tortugas. 137 



difference in the rates in sun or shade conditions and actually give off 

 a cubic centimeter of water in a very short period — i. e., in about 14 

 minutes. It is thus seen that this physiological action of loss of water 

 varies considerably, even in these plants adapted to these xerophytic 

 conditions and highly specialized morphologically. 



MARINE ECOLOGY. 



The submerged plants growing about the Tortugas are not many, 

 since marine algae most abound on rocky shores and rocks are conspic- 

 uously lacking in the Tortugas physiography. Of the higher plants 

 several interesting spermatophytes are very abundant, viz: 



(1) Thalassia testudinum Koenig and Sims, which occurs on exten- 

 sive areas in shallow water. The leaves and stems of this plant, the 

 turtle-grass, are cast up in long rows by the waves, particularly after 

 storms, on the sandy beaches of the islands. 



(2) Another component of this mass of debris is the manatee-grass, 

 Cymodocea manatorum Ascherson, which has much more slender leaves 

 and a peculiar inflorescence. It was the author's good fortune to 

 secure both of these plants in bloom, and the Cymodocea in fairly large 

 quantity by going down in a diving helmet in about 3 to 4 meters of 

 water and walking about on the bottom, making a close observation of 

 the beds of this grass. Algae were also collected in the same manner. 



(3) Two other spermatophytes, which were obtained by the deep-sea 

 dredging apparatus carried by the Laboratory yacht, were two species of 

 Halophila, growing in 17 to 19 fathoms of water out in the Gulf Stream. 

 These were H. engelmannii and H. baillonis Aschers. The occurrence 

 of these plants in this locality and their interesting relations to the 

 region have been discussed by the writer in another paper.^ These 

 and the calcareous algse which are given in a list below are quite abun- 

 dant. Representatives of the Rhodophycese and the Phaeophyceae 

 are not nearly so plentiful. The Udotea and Penicillus are quite com- 

 mon in sandy shallows and various species of Halimeda contribute a 

 large share to the detritus making up the land. Codium and Acetah- 

 ularia are frequent on rocks, shells, or other submerged objects afford- 

 ing a firm foothold. Gracillaria is mostly brought up on shells and 

 sponges in the dredges along with species of Halophila, while Sargassu7n 

 hacciferum, and occasionally Valonia, Dictyosphceria, Lynghya, Hypneo, 

 and Lawrencia, drift about the islands and in the Gulf as large mats or 

 rafts. These are washed up on the shores in long windrows, and wliile 

 floating these mats of Sargassum furnish an abiding-place for numerous 

 small animals, especially several species of crabs. One which is fairly 

 common matches exactly the yellow-brown fronds of the Sargassmn 

 and can be seen only with difficulty in a tangle of the gulfweed when 

 thrown into an aquarium. 



'Bowman, H. H. M.: Adaptability of a Sea Grass. Science, n. s., vol. xlii, 1103, 

 pp. 244-247. 



