66 ORGANISMS OF ONE CELL 



like old age, the origin of death, the significance of fertilization 

 are all deep problems which rest upon the very foundations of 

 biological science. 



Animals and Plants. At first glance it would appear to be a 

 simple matter to distinguish between animals and plants, and 

 superficial observers do not hesitate to do so with full assurance. 

 But when we come to examine them more closely we find that 

 no definite boundary line can be drawn between them, and no 

 single characteristic belongs absolutely to one or the other 

 group. Formerly it was argued that plants are quiescent while 

 animals move, and the power of spontaneous movement was 

 regarded as sufficiently characteristic to distinguish them. But 

 numerous sensitive plants, the Venus fly trap for example, and 

 many algae have the power of movement quite as well developed 

 as do many animals, while many animals, as for example the 

 sponges, are quiescent. Again it was thought that chlorophyll, 

 giving well-defined colors to plants, is a definite and distinc- 

 tive feature. But many animals such as Euglena and many 

 other flagellates are similarly colored by chlorophyll. The 

 presence of chlorophyll indicates a power of manufacturing 

 starch and sugars, while plant cells generally have a definite 

 membrane of cellulose which is closely allied to starch in chem- 

 ical composition. Cellulose therefore was also regarded as a 

 specific plant characteristic. But again a number of animals 

 have the power of manufacturing cellulose; the Dinoflagellates, 

 for example, have a thick cellulose wall, while some of the higher 

 animals, notably the group known as the ascidians, have well- 

 defined tests of the same material. Still later it was maintained 

 that plants do not eat solid food in the form of proteins while 

 animals do; but the Venus fly trap Dionaea not only moves 

 but also catches and digests insects of various kinds. On the 

 other hand, a number of animals, especially the unicellular ones, 

 do not take in solid food but manufacture it, as do the plants, 

 through the aid of chlorophyll. Similarly with every other 

 distinctive feature we find some exceptions on one side or the 

 other, showing that physiological processes in nature are no- 

 where monopolized by any one type of organisms. 



