_Protoplasm, be it in plant or animal, breathes and throws off waste materials._ When a living thing does work oxygen unites with food in the body; the food is burned or _oxidized_ and work is done by means of the energy released from the food. The waste materials are _excreted_ or passed out. Plants and animals alike pass off the carbon dioxide which results from the oxidation of food and of parts of their own bodies. Animals eliminate wastes containing nitrogen through the skin and the kidneys. _Protoplasm can reproduce, that is, form other matter like itself._ New plants are constantly appearing to take the places of those that die. The supply of living things upon the earth is not decreasing; reproduction is constantly taking place. In a general way it is possible to say that plants and animals reproduce in a very similar manner. The Importance of Reproduction.--Reproduction is the final process that plants and animals are called upon to perform. Without the formation of _new_ living things no progress would be possible on the earth. We have found that insects help flowering plants in this process. Let us now see exactly what happens when pollen is placed by the bee on the stigma of another flower of the same kind. To understand this process of reproduction in flowers, we must first study carefully pollen grains from the anther of some growing flower. [Illustration: Pollen grains of different shapes and sizes.] Pollen.--Pollen grains of various flowers, when seen under the microscope, differ greatly in form and appearance. Some are relatively large, some small, some rough, others smooth, some spherical, and others angular. They all agree, however, in having a thick wall, with a thin membrane under it, the whole inclosing a mass of protoplasm. At an early stage the pollen grain contains but a single cell. A little later, however, two nuclei may be found in the protoplasm. Hence we know that at least two cells exist there, one of which is called the sperm cell; its nucleus is the sperm nucleus. [Illustration: A pollen grain greatly magnified. Two nuclei are found (_n_, _n'_) at this stage of its growth.] [Illustration: Three stages in the germination of the pollen grain. The nuclei in the tube in (3) are the sperm nuclei. Drawn under the compound microscope.]