550 



INTERRELATIONSHIPS OF ORGANISMS 



Among the plants a somewhat comparable contrast between adapta- 

 tions for terrestrial and aquatic life occurs. The necessity for respiration 

 under drying conditions is met in terrestrial plants by a water- and 

 respiration-proof cuticle and the development of stomata, which permit 

 respiration and yet prevent undue water loss. In normally submerged 

 plants the cuticle and stomata are absent. In many amphibious plants 

 that are often partially submerged beneath oxygenless water, special 

 air ducts are developed to carry atmospheric air to the submerged parts. 



Fig. 32.6. The common rock or acorn barnacle, Balanus balaiwides — a sedentary marine 

 crustacean that casts a net to catch plankton. The greatly reduced body is attached by the 

 head and enclosed in a shell of calcified plates, with a ventral door through which the 

 feathery feet can be extended. The feet flicker in and out almost too fast to be seen, each 

 time straining out of the water whatever plankton organisms happen to be there. On the 

 left the barnacle is seen with feet extended, on the right with them coiled inside the closed 

 shell. (Courtesy American Museum of Natural History.) 



The medium and food manufacture in plants. The problems of 

 obtaining the raw materials and energy for food manufacture are not 

 greatly different for terrestrial and aquatic plants. The penetration of 

 light into the water permits some photosynthesis to a depth of perhaps 

 100 meters or more, although the great majority of aquatic green plants 

 are confined to the upper 10 to 20 meters. Carbon dioxide is present in 

 solution in nearly all natural waters and is constantly being supplied by 

 the respiration of living aquatic organisms and the decay of dead ones. 

 The other requisites for food manufacture are also in solution and are 

 available by diffusion through the permeable surface membranes of 

 aquatic plants. 



The medium and food capture by animals. Both terrestrial and 

 aquatic animals show a wide variety in the kinds of foods they seek and 



