20 THE COMPOSITION OF LIVING ORGANISMS. 



the form of flat scales. As the cells become flattened their sab- 

 stance changes. The protoplasm diminishes in quantity and dies ; 

 so that near the surface the cells are wholly dead, and finally fall 

 off.* In a similar manner are formed the lifeless parts of nails, 

 claws, beaks, feathers, and many related structures. A hair is 

 composed of cells essentially like those of the skin. At the 

 root of the hair they are alive, but as they are pushed out- 

 wards by continued growth at the root, they are transformed 

 bodily into a dead, horny substance forming the free portion of 

 the hair. Feathers are only a complicated kind of hairs and are 

 formed in the same way. 



It is a significant fact that the quantity of lifeless matter in 

 the organism tends to increase with age. The very young plant 

 or animal probably possesses a maximum proportion of proto- 

 plasm, and as life progresses lifeless matter gradually accumulates 

 within or about it, sometimes for support, as in tree-trunks and 

 bony skeletons ; sometimes for protection as in oyster- and snail- 

 shells ; sometimes apparently from sheer inability on the part of 

 the protoplasm to get rid of it. Thus we see that youth is the 

 period of life and vigor, and age the period of comparative life- 

 lessness, in a literal and material as well as in a figurative sense. 



Summary. The bodies of higher animals and plants are hete- 

 rogeneous in structure and function, and are composed of organs. 

 These may be resolved into one or more tissues, each of which 

 consists of a mass of similar cells (or their derivatives) having a 

 similar function. The cells are small masses of living matter, 

 or protoplasm, which deposit more or less lifeless matter either 

 around (outside) them or within their substance. In the former 

 case the protoplasm may continue to live, or it may die and be 

 absorbed. In the latter case it may likewise live on for a time, or 

 may die, either disappearing altogether or leaving behind a resi- 

 due of lifeless matter. 



* It is probable that this process is connected with the deposition within 

 the protoplasm of extremely minute granules of lifeless matter (eleidiii). 



