152 HYDROPHYTES sect, iv 



Brandt ^ asserts that the sea is rich in nitrogen, which is constantly 

 being suppUed to it, and is reduced by denitrifying bacteria. This process 

 is more active in tropical than in temperate seas, and partly for this 

 reason the ocean in sub-tropical and tropical regions is relatively poor in 

 organisms, while it is rich in them in cool and cold regions. 



Many waters contain organic compounds which, by consuming the 

 oxygen, render water unsuitable for the existence of autophytes. 



The nutritive substances most important to plants, such as potassium, 

 phosphoric acid, ammonia, and sulphur, occur in all water, but only in 

 small quantities and in an extreme condition of dilution ; in fresh water 

 compounds of potassium and nitrogen are present in far greater propor- 

 tions than are the other compounds in question. But we have no know- 

 ledge that these conditions have any distinct effect upon the distribution 

 of aquatic plants. Certain desmids and diatoms are stated to prefer lime, 

 others silica ; similar minor differences are attributed to other plants. 

 Common salt, on the contrary, is of profound importance. In sea-water, 

 among the numerous salts, including chlorides of sodium, magnesium, 

 and potassium, also sulphates of magnesium and calcium, the first named 

 is of far the greatest significance. The amount of common salt in the 

 ocean fluctuates within only narrow limits, but that in smaller seas 

 varies greatly, not only in different sites, but also in the same place at 

 different times. The following are the approximate percentages : 



Red Sea 4, Mediterranean Sea 3-5-3-9, Pacific Ocean 3-5, Skager-Rak 3, 

 Kattegat 1-5-3, the Great Belt 1-27, the Oresund 0-92 (in the last two, very 

 variable according to the currents), Gulf of Bothnia o-i-o-5, Gulf of Fin- 

 land 0-3-0-7. These statistics relate to superficial water ; but in parts of 

 the sea round Denmark at a greater depth there is a saline undercurrent 

 from the North Sea. 



The great difference between the floras of salt and fresh water will 

 be discussed later in this Section. 



Although not a few fresh-water algae, especially lowly organized forms, 

 can adapt themselves to the presence of common salt, which causes an 

 enlargement of their cells and other formal changes, yet there are no 

 plants other than certain diatoms that are common to fresh and slightly 

 saline water ; nevertheless, in the brackish water of the Baltic Sea there 

 grow some Characeae, Enteromorpha intestinalis, and Potamogeton 

 pectinatus, which also occur in fresh water. 



The cyanophyceous communities occurring in special places will be 

 dealt with later. 



Specific gravity of water. Salt water and fresh water differ greatly 

 in specific gravity, and consequently in buoyancy, which plays a great part 

 in connexion with plankton-organisms ; for fresh water, as is well known, 

 has a smaller power of keeping bodies from sinking. The regular seasonal 

 changes in temperature of fresh water bring in their train corresponding 

 ones in the specific gravity and viscosity of the water. Many plankton- 

 organisms undergo periodic changes of shape, which all seem to be in the 

 direction of increasing the resistance-surface, and which synchronize with 

 the changes of temperature. It therefore seems highly probable that 

 these seasonal variations in shape are to be regarded as responses on 



* Brandt, 1904. 



