Effects on Structure and Size 33 



small organisms and such animals as earthworms and slugs can main- 

 tain their shape without skeletal material of some sort. The woody 

 tissue characteristic of the higher plants provides rigidity against the 

 force of gravity. The bones and muscles of the larger land animals 

 are similarly arranged primarily to provide support (Thompson, 

 1942). 



Generally speaking, the weight of an organism tends to increase as 

 the cube of its linear dimensions, but the strength of supporting 

 columns increases only as the square of the dimensions. As a result 

 animals and plants are definitely limited as to size. Since land plants, 

 once established, do not require locomotion, they can have a much 

 larger amount of rigid supporting tissue than animals. Hence the 

 plant kingdom holds the record for size on land. The giant redwoods 

 of Cahfornia (Fig. 1.1) attain heights well over 100 m (record height: 

 365 ft), and the trunks alone are estimated to weigh as much as 500 

 or 600 tons. In the animal kingdom few modern species attain a size 

 as great as 6 or 7 tons, although the dinosaurs of the past were some- 

 what larger. Possibly Brontosaurus would have tipped the scales at 

 30 or 40 tons, but Brontosaurus came to an unhappy end, no doubt 

 in part because of its ungainly size. 



In the water environment, since all parts of the organism tend to be 

 buoyed up by the medium, supporting structures may be greatly re- 

 duced or entirely lacking. Woody tissue is needed for support by 

 few aquatic plants. When an elaborate skeleton is present in aquatic 

 animals, it usually occurs for purposes other than support. For many 

 crustaceans and mollusks the hard tissues serve primarily as pro- 

 tection; in other forms, such as fish, the skeleton is used chiefly for 

 the attachment of muscles of propulsion. Many aquatic organisms 

 such as the jellyfish have no skeleton at all. It is true that the jellyfish 

 is a weak and sluggish organism but the octopus and the giant squid 

 are decidedly vigorous, and yet the skeletons of the latter are reduced 

 to horny pens and a few cartilages in the head region. An octopus 

 kept in an aquarium at the Bermuda Biological Station was so suc- 

 cessful in getting out of his tank in spite of a weighted lid that he was 

 named Houdini. 



In the plant kingdom even the algae can grow to tremendous sizes. 

 The giant kelp Nereocystis, common off the west coast of the United 

 States, may grow to a length of more than 35 m, and Macrocystis is 

 reported to attain an even greater size. Such plants have no woody 

 tissue whatsoever, but the water buoys up the extended parts of the 

 organism. Animals in the sea today attain sizes larger than ever 

 existed on land. By weighing the parts of a blue whale being cut up 



