Page:Popular Science Monthly Volume 49.djvu/630

608 that, in any case where a wall does exist, it consists of true cellulose.

The protoplasm of a cell is usually bounded at its outside by a denser hyaline layer which is very impermeable for fluids while living, and thus serves as a protection against the penetration of foreign or harmful substances into the cell. But this outer layer appears also to be the receptive portion of the protoplasm which is sensitive to and transmits external stimuli. Within it is the rather fluid, undifilerentiated, granular protoplasm which constitutes the basis of the cell, and in which lie the special organs of which we have spoken. This wonderful mixture of albuminoid or proteid substances, which has well been called "the physical basis of life," must therefore possess those fundamental properties of living things, the power to assimilate, to grow, and to respond to external stimuli; and it is easy to show that living protoplasm possesses all these properties. But the one which most interests us just here is that of assimilation. This power of converting food into its own substance, which may result in the increase of that substance, or growth, seems specially to belong to the granular protoplasm, which may be regarded as the nutritive organ of the cell. Just here we note that the food which may thus be assimilated must be organic substance. It may be proteid, like albumin, casein, or fibrin; it may be a carbohydrate, like sugar or starch; it may be a hydrocarbon, such as fat or oil; but organic it must be. Whence comes now the supply of food? Plainly, in most cases, by absorption from without. In animals the solids and fluids taken in are reduced by digestion to the fluid form, and are then transported to the various cells of the organism, to be absorbed and assimilated by them. In those plants known as fungi, which can develop only on living or dead organisms, the food materials are absorbed in fluid form, being sometimes first reduced to that form by the action of a ferment secreted by the fungus. But certain cells of most plants have the power of manufacturing their own food from inorganic materials, and thus of living independently of other living things. Thus the green plants bridge over the chasm between the inorganic and the organic, and the life of all organized beings is practically contingent on their life. In the granular protoplasm of some cells of these plants may be found differentiated protoplasmic masses which contain the green pigment chrophyll, that gives them their color. First recognized early in the present century, these masses were observed by Naegeli, in 1846, to increase by division, and therefore to constitute living organs of the cells in which they occur. These chlorophyll bodies possess the synthetic power of recombining the elements of simple compounds obtained from the air and the soil, in the presence of light, into complex organic