Mr. Worsley-Benison has been discussing this question in a very interesting way, and he says in conclusion that "physiologically the most distinctive feature of plant-life is the power to manufacture protein from less complex bodies; that of animal-life, the absence of such power." He finds that in form, in the presence of starch, of chlorophyl, in power of locomotion, in the presence of circulatory organs, of the body called nitrogen, in the functions of respiration and sensation, there are no diagnostic characters. He finds, however, "fairly constant and well-marked distinctions" in the presence of a cellulose coat in the plant-cell, in digestion followed by absorption, and in the power to manufacture protein.
The morphological feature of plants is this cellulose coat; of animals, its absence; the physiological peculiarity of plants, this manufacturing power; of animals, the want of it. But after all the discussion he says: "To the question, Is this an animal or a plant? we must often reply, We do not know. - The Microscope.