Growth and Movement. 133 



season ; these are known as biennials. Still other plants 

 keep in reserve sufficient energy to maintain their indi- 

 vidual existence year after year ; these are termed perefi- 

 nials. The production of seeds is in the interest of the 

 species, but not of the individual, upon which it is a severe 

 tax, and only those plants can survive it that can retain 

 within their own bodies a good amount of reserve energy. 

 Some perennials further protect the individual life by 

 alternate years of seed-bearing and sterility. 



106. Length of Life. — Some perennials attain an enor- 

 mous age. A Taxus tree in Kent is considered to be 3000 

 years old, and an Adansonia in the Cape Verde Islands 

 and a Taxodium in Mexico appear to have lived for 6000 

 years. It must be remembered, however, about plants of 

 great age, that those of their tissues which are actually 

 alive are of comparatively recent production ; the older 

 tissues are gradually dying, while new tissues are being 

 formed. Since perennial plants are annually rejuvenated 

 by the formation of new tissues, it would seem that they 

 might live indefinitely ; but changes in the character of 

 the soil, the ravages of storms and parasites, and com- 

 petition with other individuals sooner or later bring their 

 life to an end. 



107. Nature of Growth. — If we were to attempt a formal 

 definition of growth, we might say that it is any permanent 

 change in the form and size and internal structure which is 

 brought about by the action of the protoplasts. Growth 

 does not always involve an increase in weight, for germi- 

 nating seeds are actually decreasing in weight by the oxida- 

 tion of their reserve materials ; neither could temporary 

 increase in size by the imbibition of water be classed as 

 growth. 



