PELAGIC PLANT LIFE 367 



pass more rapidly through their life-cycle, and their numbers may 

 be more drawn upon by the abundant animal life ; consequently 

 considerable additions to their apparent total may be necessary, 

 if we wish to estimate properly the importance of plant life 

 in the tropics, as compared with that in higher latitudes. We 

 must remember, moreover, when dealing with observations 

 made in coastal waters all the year round, that the different 

 species have a natural periodicity that may be connected 

 with unknown internal factors in their cycle of life, as well 

 as with the influence of currents which at one time carry the 

 surface - layers away from the coast and at another time 

 towards it. All the same there are many irregularities which 

 cannot be explained as being solely the result of the actual 

 physical conditions of existence. Besides light and warmth we 

 might perhaps be apt to think of salinity, which, in the course of 

 its variations, influences both the density and the osmotic tension 

 of the sea-water. Though we are aware that a low or greatly 

 varying salinity is injurious to many pelagic organisms, there 

 are others which thrive remarkably well and multiply exceed- 

 ingly under such conditions, as for instance the diatom 

 Skeletoneina costahmi and the peridinean Ceratiurn tripos forma 

 subsalsa. Results, in fact, are often the reverse of what one 

 might expect. The flora of brackish - water bays, which is 

 poor in species, may develop into even greater masses than we 

 find synchronously in the open sea, where no osmotic changes 

 have disturbed the vital activity of the numerous species 

 belonging to the community of oceanic algse. 



We cannot get away from the view, which was first con- Brandt. 

 fidently put forward by Brandt, that certain indispensable 

 nutritive substances occur so sparsely that, according to Liebig's Liebig's 

 minimum law, they act as factors which limit production. ™™'"""^ ^^• 

 Liebig found that the growth of plants on land depends on the 

 amount of the requisite nutritive substances present, the deter- 

 mining substance being the one of which at any moment there 

 is least in proportion to the needs of the plant. As long as 

 a particular nutritive substance occurs " in minimum," plant 

 production will be proportionate to the available quantities of it, 

 even though there be a superabundance of all other essentials. 



If this law is made to include all necessary conditions of life, 

 it will be found to apply universally to all organisms both on land 

 and in the sea, in which case that condition of existence, whether 

 it be physical or chemical, which occurs " in minimum," will be the 

 factor of limitation. We must remember, however, that produc- 



