16 Plant Life and Evolution 



We have but to compare a tree like the oak, for 

 example, with a highly specialized animal like the 

 horse. The former may live for centuries, produc- 

 ing each year thousands of leafy twigs and flowers, 

 each like its neighbors. The complexity of the plant 

 is due to the multiplication of similar organs rather 

 than to a great number of different parts. The 

 animal is sharply individualized, its different organs 

 being absolutely definite in number and position 

 with only a limited power of regeneration. It has 

 a very limited and definite period of growth, with 

 a correspondingly brief life-span. 



Regeneration in Plants. The power of regenera- 

 tion in the higher animals is very limited. A 

 wound may heal, and such organs as nails, feathers, 

 and hair may be replaced, and in some lower verte- 

 brates even a whole limb or tail may be regenerated. 

 But in the higher forms of animals, the regenera- 

 tion of whole individuals, except sexually, never 

 occurs. In plants, on the other hand, even in the 

 highest ones, the regeneration of the whole indi- 

 vidual from almost any member of the body is 

 possible, and this fact is constantly taken advan- 

 tage of in the artificial propagation of plants by 

 cuttings, grafts, etc. In some cases a fragment of 

 a leaf or root is enough for the development of a 

 complete plant, and in many of the lower plants, 

 a single cell is sufficient. Plants also may show a 

 regular periodic regeneration of certain organs, such 

 as leaves and flowers, which are short-lived and 



