DEPARTMENT OF MARINE BIOLOGY. 191 



One phase of these mortahty experiments which was not repeated this 

 year was that of Rhizophora cultures in superconcentrated sea-water; 

 neither were transpiration-rate records taken for such cultures this season. 

 Not only were the leaves of Rhizophora tested by the Stahl method, but 

 the author devised a special instrument for the purpose of getting the rate 

 of transpiration on the long, pendulous prop-roots of older trees. This 

 special clasp is made to contain the indicator-paper just as in a Ganong 

 clasp, but was designed to fit the cylindrical contour of the roots and is 

 operated by a spring which is, in the author's estimation, an improvement 

 on the older type of instrument. A more detailed description of this instru- 

 ment and the results of these tests on old trees, taken in situ, will be given 

 later and in a fuller manner than can be here presented. As there were no 

 old trees at Tortugas, these observations were made along the Miami River 

 and in Biscayne Bay, together with physiologic observations of a different 

 character. The above region offered an ideal locality for observing the 

 transpiration-rate through the lenticels of the pneumatophore prop-roots of 

 the old trees, because the river and bay together afford all the varied con- 

 ditions from fresh to salt water. These results also will be tabulated and 

 published in the full paper dealing with this work. 



Second. — Besides the work just noted, the latter portion of this past season's 

 work was the gathering of material to finish the author's paper on the botan- 

 ical ecology of the Tortugas region, which will appear as a contribution from 

 the Tortugas Laboratory. This work was begun in the early part of 1915, and 

 specimens were taken and observations made at intervals since that time. 

 The peculiar climatic conditions of the group make a study of the flora of 

 this region most interesting. The vegetal sequence in these islands and the 

 factors in dissemination are also worthy of even more intensive study than 

 the investigator has been able to devote to them in the few weeks the Labora- 

 tory is open during the year. The writer used a portion of the latter half 

 of the season in making a series of carefully plotted distributional maps, 

 with the aid of a plane-table and sighting-rule. These will aid in illustrating 

 the factors concerned with dissemination in these islands. 



Among the observations pertaining to this work a considerable number 

 of potometric records were taken on all the main species of the Tortugas 

 flora, such as Tournefortia, Suriana, Canavali, etc. The transpiration 

 records were taken with a graduated Ganong potometer under several con- 

 ditions, as sun and shade, etc. This data is very striking and illustrates the 

 extreme adaptation of these plants to conserve moisture. 



Supplementing the observations on the spermatophytes of the region, a 

 full collection of the marine algae of the reefs has been made, both herbarium 

 specimens and material in liquid preservatives being secured. This collec- 

 tion was made possible by the use of a diving helmet and also by the utiliza- 

 tion of the deep-sea dredging apparatus with which the Laboratory's yacht 

 is equipped. The list and distribution of these algae will probably be used 

 in connection with the ecologic paper. 



While dredging out in the Gulf of Mexico, on the edge of the Gulf Stream, 

 in 12 to 17 fathoms of water, an effort was made by the writer to again secure 

 specimens of that curious little submerged spermatophyte, Halophila, which 

 the author described in a recent publication as occurring at unusual depths. 

 Hopes were entertained that the staminate form might be collected, but 

 the expedition was without success in this, though plenty of material was 

 collected of two species of Halophila to pursue some further research on 

 this interesting genus. 



