18 PLANTS AND MAN 



end, to better satisfy the inherent demands of heterotrophic 

 animal metabolism. Plants have protoplasm which is also sensi- 

 tive and irritable, but have developed these characteristics to 

 only a slight extent and have survived in the struggle for existence 

 without them. Plants feel, but in a limited sort of way; plant 

 protoplasm is sensitive to gravity, pressure, chemicals in solution, 

 temperature, light — any factors in the environment which are 

 important in the living routine of the plant organism. Any of 

 these may bring about a response on the part of the plant, which 

 though slow and inconspicuous is still essentially a response to 

 a stimulus. 



Growth and Reproduction 



There are other activities of protoplasm which indicate a 

 kinship between the two kingdoms of life, of which we might 

 mention growth and reproduction. When a cell has assimilated 

 more food than it needs, it uses some of it to increase the amount 

 of protoplasm within the cell; but soon the cell reaches its maxi- 

 mum and very definitely circumscribed proportions. At this point 

 the cell divides in half, a new cross wall forms, and two cells 

 result. This growth of an organism by cell division is the method 

 by which all plants and animals increase in size as they develop 

 from minute beginnings to maturity. The acorn becomes a 

 massive oak by cell division. There are slight variations in this 

 process in the two kingdoms, however. Animal growth results 

 in more definite and limited proportions characteristic of each 

 species, while the size of a plant varies within wider limits. In 

 addition, certain parts of the plant body have unlimited growth. 

 This is true of the growing tips of roots and stems and of the 

 growth ring beneath the bark of woody plants. The latter is a 

 layer of potentially immortal cells which causes growth in diame- 

 ter, bringing about possibilities of annual increase in girth limited 

 chiefly by the restrictions of the environment. Some Sequoias 

 (fig. 7) have been growing for over five thousand years and may 

 continue to do so for another five thousand. Such unlimited 

 growth gives plants the ability to live to far greater age than 

 animals can, many common trees exceeding a century of life. 

 Another variation in the growth process is associated with the 



