412 MARliNE PLx\NTS. 



with its torn off lea^'es. It is not advisable to walk over 

 these heaps, for they gi^'e wa\' under the weight of the bod^' 

 and one sinks deeply- into tlie wet leaves. To the west of 

 the Cap, on the sands of Golfe Jouan, round balls of a 

 light brown colour and librous structure are often found. 

 These used to be seen in chemists' shops under the name 

 of "pilae marinae". The^' are loose pieces of the root- 

 stock of (irass-wrack covered with the fra^-ed remains of 

 leaves. These are tossed about on the beach b\' the 

 waves and formed into balls a decimetre in circumference. 

 Plant life in the sea does not descend lower than 

 1,150 feet. Below 2(:)0 feet it decreases rapidh- even in 

 the southern seas, for at this depth the light is too dim 

 to maintain vegetable gro'VA'th. By means of photographic 

 plates Fol and vSarasin proved that in the Mediterranean 

 sea, eighteen miles from the coast of the Riviera, there 

 is still a xevv faint light action at a depth of l,5,-!0 feet. 

 Other investigators state that this action of light can, 

 under certain conditions, exist 3(i0 feet lower down. But 

 the assimilation of carbon b\' the plant has long before 

 this become impossible, especialh' as the composition of 

 the light changes with the depth. Of tlie various col- 

 oured ra\s of the spectrum, the red and ^-ellow are first 

 absorbed, and the blue and \'iolet last. If a beam of 

 sunlight be allowed to pass through a tube a yard and 

 a half long filled with pure vi-ater, the red colour will 

 liave already completeh' disappeared, the ^'ellow will be 

 but feebh' represented, and the green will be the most 

 brilliant. .Xear the surface, in sea water, green and blue- 

 green ra\s predominate, while the red and \'cllow have. 



