Page:Popular Science Monthly Volume 21.djvu/301



CHILD'S toy-balloon may afford us an illustration of what a naturalist might call a typical cell. We have in the toy simply a closed sac thoroughly distended by its contents, more or less perfectly spherical in shape, and affording in outline or cross-section an almost perfect circle. In the organic cell the sac is known as the cell-wall, and whatever may be inclosed by the cell-wall is called the cell-contents. A typical cell would be round, spherical, but very few cells, as they occur in nature, are perfect spheres. A cell which may be spherical at the outset may change its shape in accordance with changing circumstances, so that we may say that the form of all cells which we find united to form tissues varies with the situation which such cells occupy, and the functions of the tissues themselves. This we shall see more clearly as we go on. That vegetable tissues, as they occur in wood, pith, leaves, flowers, and fruit, are entirely composed of cells, may be easily demonstrated. All that is needed is, to take a very thin slice of any of these substances and examine with a microscope of moderate power, when the cellular structure becomes immediately apparent. So, then, all the great variety of form and color, and all the resulting beauty, which the vegetable kingdom affords, and all the varied economic value of plants, depends upon the form and contents of these little organic units—of cells. More than this: these cells are of the highest scientific interest. All the discussion of the past few years in regard to spontaneous generation and the origin of life has been a discussion of vegetable cells; and very much of all that we know about life, its activity and its mystery, has been derived from