THE EVOLUTION OF PLANTS 



1. Ability to take in matter from without and transform 

 it into matter like themselves. This we call metabolism and 

 nutrition. 



2. Ability to groiv — to increase in size and weight. 



3. Ability to reproduce their kind. 



4. Ability to detect changes in their surroundings and to 

 react or readjust themselves to the changed conditions. This 

 we speak of as ability to detect and to respond to stimuli. 



It is also probable that the earliest forms of life were so 

 simple that they could be regarded as neither plants nor 

 animals, but merely as organisms or "living things." Such 

 organisms are well known today. The viruses, which cause 

 the so-called virus diseases of plants, are possibly of this 

 nature. They behave like living things, but they are so small 

 that they cannot be seen with the most powerful microscope. 

 This means that their greatest dimension is less than one-half 

 the wave-length of light. 



One group of organisms, known as slime molds (Myxo- 

 mycetes. Fig. 2), at one stage of their existence so closely 

 resemble the tiny animals (animalcules) known as Amoebae 

 (singular, Amoeba^ that they can hardly be told apart. Both 

 are naked bits of protoplasm, capable of motion and 

 locomotion. Zoologists have regarded them as animals; 

 botanists have contended that they are plants. 



The similarity between animals and plants in their essen- 

 tial life processes has long been recognized by biologists and 

 was forcibly presented by Claude Bernard in his classical 

 Legons sur les Phenomenes de la Vie Communs aux Animaux 

 et aux Vegetaux (1878-79) . In the processes of respiration, 

 digestion, cell-division, growth, reproduction, transmission 

 of heritable characters, the possession of irritability and the 

 power to detect and respond to stimuli, and in other physi- 

 ological processes, they are essentially alike. 



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