xvi Life and Living Creatures 311 



involves nutrition or the taking in of food -substance, 

 digestion or the elaboration of food, and assimilation or 

 absorption into protoplasm. While this process goes on the 

 organism grows by the assimilation of unlike substances, 

 which are transformed into protoplasm and added to the 

 mass from within and throughout. The simultaneous 

 breaking -down process, on the most commonly accepted 

 theory, is brought about by respiration or the absorption of 

 oxygen. Protoplasm is an extremely unstable compound, 

 always ready to combine with oxygen and break up into 

 carbonic acid, water, and a very small proportion of a few 

 other stable compounds. The living protoplasm is purified 

 by the process of excretion, which is simply the thrusting 

 out of the burnt products (carbonic acid, water, etc.) and 

 of those parts of the food which escape digestion. When 

 life ceases, protoplasm ceases to grow, oxidation continues 

 unchecked, and the organism breaks up and decays away 

 by slow combustion. In the process of growth, matter 

 which is not living may be built into the substance. For 

 example diatoms and radiolarians, which are single-celled 

 organisms, form coats or skeletons of silica, and foramini- 

 fera, also consisting of one cell, secrete hard shells of 

 carbonate of lime ( 273). All organisms, except the 

 protozoa and the simplest plants, consist of many cells 

 containing protoplasm, built up into organs set apart for 

 special purposes. These cells are usually supported in a 

 framework of matter such as wood or bone, elaborated 

 by the living organism and sharing its life for a time, but 

 becoming practically lifeless as they grow older. When a 

 cell grows, it increases in size to a certain limit and then 

 divides into two cells, the process being termed reproduc- 

 tion. In the protozoa the division of a cell is complete 

 separation, producing two individuals ; but in higher organ- 

 isms a single cell, termed an ovum or egg-cell, is separated 

 from the rest, and grows by subdivision into a separate 

 many-celled organism similar to the parent form. Most 

 often, both in plants and animals, this liberated cell is unable 

 to develop until it unites with a cell of another kind (termed 

 a male cell) from the same species. Thus the continuance 



, UNIVERSITY 



