112 THE SEAS 



and the word is now in general use among those interested 

 in the science of the sea. 



Plankton Plants 



One naturaJly wishes to know what kinds of creatures 

 these are that make up the almost infinite multitudes that 

 drift freely throughout the water layers. The plankton is 

 now known to play a part of the greatest importance in the 

 economy of the sea, and in this respect the organisms that 

 deserve our first consideration are the microscopic plants. 

 These are not like the plants on land, but consist generally 

 each only of a single cell ; nevertheless, they are plants in 

 the true sense of the word, because each contains within its 

 cell colouring matter closely similar to that so characteristic 

 of our land vegetation. These little plants are known as 

 " diatoms " ; they are so called because they have, sur- 

 rounding their cell-walls, glass-like protective shells com- 

 posed of two halves — two lid-like structures that fit one 

 into the other, and thus enclose the body of the plant in a 

 little box. 



In order to catch these diatoms it is necessary to use a 

 net made of the finest muslin or silk, as they are, mostly, so 

 small that they will pass through the meshes of ordinary 

 coarse muslin. The catch, to the naked eye, will look like 

 a greenish -brown scum, and if placed under the microscope 

 will be seen to consist of a jumble of interlacing spines 

 amongst which may be noticed green oblongs, squares and 

 circles. To find out the true nature of the catch it must 

 be diluted with sea water and only a drop examined, when 

 it will be found that the diatoms are much fewer in number 

 and separated one from the other so that their true structures 

 can be made out. Some will be like Uttle circular discs, 

 others oblong with little spines or horns projecting from each 

 corner, and others strung together to form chains of tiny 



