This process of growth can take place only when living protoplasm is present in the cells of the plant. The substance we call protoplasm is an albuminoid, like the white of an egg, and it forms the flesh of both plants and animals. A living plant can assimilate its own protoplasm, an animal must take it ready-made from plants. But a plant can assimilate its food and grow only under the mysterious influence we call life. Life alone brings forth life, and we are as far as ever from understanding its nature. Around our little island of knowledge, built up through the centuries by the labor of countless workers, stretches the infinite ocean of the unknown.
Gray's First Lessons. Sect. VII, XVI, §2, §4, §5, §6, 476-480.
How Plants Grow. Chap. I, 119-153, Chap. III, 261-280.