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 grow— o 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 (Myxomycetes. 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. Zoölogists have regarded them as animals; botanists have contended that they are plants.
The similarity between animals and plants in their essential life processes has long been recognized by biologists and was forcibly presented by Claude Bernard in his classical Leçcons sur les Phénomènes de la Vie Communs aux Animaux et aux Végétaux (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 physiological processes, they are essentially alike.
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