312 



DEPTHS OF THE OCEAN 



ground system of roots to collect moisture and nourishment 

 from the soil, and its leaves are raised aloft on slender stems 

 to derive benefit from the rays of light and build up organic 

 substance out of carbonic acid and water. Ocean plants have 

 no such point cTapptii \ they find their nourishment dissolved in 

 sea-water and distributed uniformly all around them, and they 

 get most benefit from the sunlight when they are regularly 

 spread throughout the whole bulk of the water in the photic 

 zone. Their diffusion is also their best defence against their 

 enemies, for, while animals have no great difficulty in 

 finding and consuming the larger plants, these creatures, 

 scattered everywhere like dust amidst the immeasurable 

 water-masses, are not so easily available. The majority of 

 the floating plants pass their lives as single cells, though they 

 are frequently far more highly organised than the single cells 

 that go to form a higher plant. 



As pelagic algse have generally a greater density than the sea- 

 water in which they live, they would sink out of range of the 

 rays of light, and perish, if it were not for the fact that they are 

 kept from descending either by their own exertions or by 

 suspension organs which act as a parachute. The most notice- 

 able features in their organisation are their different forms of 

 structure, which are directly connected with the floating existence 

 they lead. In what follows I shall describe the most important 

 types, belonging to a limited number of classes, most of which 

 have variously shaped pigment granules or chromatophores, 

 consisting of brown colouring matter instead of green chlorophyl. 

 Comprised in their number are diatoms, peridineae, and brown 

 flagellates, amongst which last we also include calcareous 

 flagellates or coccolithophoridse. In addition there are a few 

 pelagic representatives of the green and blue-green algse, which 

 I will discuss separately. 



A diatom can be distinguished from other algse by its 

 silicated cell-wall. This is composed of two quite similar 

 halves, or valves as they are called, that are united to one 

 another like the top and bottom of a pill-box (see Fig. 212). 

 Inside the valves the protoplasm lines the wall like a thin sort 

 of bladder, while the nucleus is frequently in the very centre 

 surrounded by a denser mass of protoplasm connected to the 

 bladder by bridges or strings. The rest of the cavity is full of 

 a clear cell-fluid. The pigment granules, which are organs of 

 nourishment, enable the diatom to collect rays of light and build 



