44 The Medium 



are the Gulf Stream, which carries water of tropical origin northward 

 along the east coast of the United States and thence northeastward 

 toward northern Europe, and the Japanese Current, which brings 

 relatively warm water to the Aleutians and the Alaskan coast. 



Sessile Existence 



Since both air and water are practically always on the move, the 

 possibility exists that animals and plants might live a lazy existence, 

 remaining in one place and letting the medium bring their needs to 

 them. The desirable features of such a tranquil life might appeal 

 to many of us. But since air can carry only the smallest and lightest 

 particles, no free-living animal on land can obtain sufficient solid food 

 by air transport. There are no completely sessile animals in the ter- 

 restrial environment. A few land animals, such as the spider and 

 the ant lion, lie in wait for the prey which flies or crawls to their 

 traps, and other forms, such as the dung beetle and the wood borer, 

 live within their food material, but even for these animals some loco- 

 motion is necessary. 



The needs of land plants, on the other hand, are such that most of 

 them can be brought by mobile agents. The carbon dioxide and 

 oxygen exchange of the plant is readily taken care of by the movement 

 of the air. Mineral nutrients needed by the plant are also carried 

 to it, but in most situations these materials are brought by the soil 

 water so that we are really dealing with a special case of the mobility 

 of water. However, certain epiphytes, such as the bromeliads and 

 Spanish mosses, live in the crotches of trees or even on telephone wires 

 where absolutely all their needs reach them by means of air transport. 

 These "air plants" obtain mineral nutrients from rain water and from 

 dust particles which lodge in their crevices. For many species the 

 wind plays an essential role also in reproduction and in distribution. 



In the water environment the transport by the medium of the needs 

 of both plants and animals is quite an ordinary occurrence. Most 

 multicellular plants in aquatic habitats lead a completely sessile exist- 

 ence, allowing the water to bring them the oxygen, carbon dioxide, 

 and food materials that they require. The algae absorb their nutrients 

 directly from the free water, but the vascular plants generally obtain 

 these materials through their roots from the water in the mud. 



Large numbers of aquatic animals enjoy a sedentary life. Many 

 groups of sessile animals, such as the sponges, coelenterates, bivalves, 

 and barnacles, are extremely abundant in the water environment, but 

 are entirely unrepresented on land. These forms can remain per- 



