104 PLANTS, SEEDS, AND CURRENTS 



and hydrometer, is here opened up, and it presents many very com- 

 plex problems. It would be futile, however, to speculate now on 

 the various working values to be assigned to the increase in density 

 due to the fall in temperature and to the increase arising from dis- 

 solved saline materials. We will leave the problems offered by this 

 contest between the elements to the future investigator, and will 

 content ourselves with the reflection that much light will be thrown 

 on the conditions which determine the stations of plants in a tidal 

 estuary by such a systematic employment of the hydrometer and 

 thermometer as was exhibited in Prof. Harshberger's investigations 

 on the estuarine plants of the coast of New Jersey. 



The Springs of the Black River Morass, as Illustrated by 

 those of the Blue Hole. — It is likely that the numerous springs 

 of fresh-water which well up in different parts of the Great Morass 

 supply a large proportion of the water that the Black River dis- 

 charges into the sea. One of the best known of these springs is the 

 Blue Hole, which lies, as the bird flies, four or five miles from the 

 coast, and is reached by way of the Salt Springs tributary, of which 

 it is one of the main sources. These springs, which are distant about 

 a mile from the foot of the neighbouring range of hills, issue in a 

 funnel-shaped, well-like hole about forty feet deep and sixty feet 

 across. The considerable body of water which here escapes is carried 

 away in a stream, the channel of which is almost blocked by aquatic 

 plants, that grow here in great luxuriance. On January 7, 1907, at 

 noon, the temperature of the Blue Hole was 75-5-76° F. at the 

 bottom, and 77° at the surface, that of the river below, which received 

 its waters, being 76-77°. 



A rich growth of aquatic and semi-aquatic plants is characteristic 

 of the large and copious head-springs of rivers in most parts of the 

 world. Here under comparatively uniform thermal conditions, 

 where the yearly range of the water-temperature often corresponds 

 approximately to the limited range of the monthly means of the 

 air-temperature in the shade, the aquatic plants thrive vigorously 

 throughout the year. For the whole twelve months it is probable 

 that the range of temperature of the water in which the vegetation 

 around the Blue Hole is bathed would be only 75 to 82° F., which is 

 the usual range of the monthly means of the temperature of the air 

 in the lowlands on the south side of Jamaica. (See a paper by the 

 present writer on the temperature of springs in the Journal of the 

 Royal Meteorological Society, about 1895, and papers on river- 

 temperature in the Proceedings of the Royal Physical Society of 

 Edinburgh, 1896.) 



The plants grow in greatest profusion in the stream leading 

 immediately from the Blue Hole. Largely occupying its channel 

 and often carpeting the bottom are masses of Isnardia palustris, 

 Hydrocotyle umbellata, Potamogeton plantagineus, etc. There also 

 flourish here in the shallows Sagittaria lancifolia and a pretty yellow- 

 flowered Utricularia. Pistia occidentalis fills up little recesses in the 

 oozy banks, propagating its kind vegetatively with rapidity. In the 

 river below Ceratophyllum demersum grows in the shallows. 



The Vegetation of the Black River District. — This may be 



