Plant cells are not independent units as assumed in the cell theory of organic structure, as recent investigations, with improved microscopes and more exact methods of research, have shown that the protoplasm of adjacent cells is connected by slender threads which pass through minute openings in the cell walls, and this has been observed in so many cases that the continuity of their protoplasm is believed to be the rule in the structure of plants. The various tissues and cells of the higher plants have, therefore, a common bond of union in the connecting threads of protoplasm which determine their harmonious action.
The higher powers of the microscope likewise show that the protoplasm of plants is not homogeneous, but contains numerous granules which repeat themselves indefinitely by a process of self-division, each granule having a genetic relation to pre-existing granules of the same kind. Besides the granules, each protoplasmic cell has a nucleus which in the same manner is formed by the self-division of a pre-existing nucleus. The granules, and especially the nucleus, may prove to be important factors in the perpetuation of ancestral characters, and consequently more intimately involved than other elements in the grand mystery of life.
The chlorophyll granules which constitute the green coloring matter of plants were supposed to be formed from the protoplasm in which they appear; but they are now known to arise from the pre-existing self-propagating granules of protoplasm.
The conception of ascending steps of constructive metabolism resulting in the formation of protoplasm and the storing of energy, with correlated descending steps, by which protoplasm is transformed into less complex compounds (destructive metabolism), with a liberation of energy, serves as a key to the complex processes of nutrition which enables us to trace their conformity to general laws that are readily recognized, and clears up the obscurity arising from the multiplicity of details which from other points of view could not be brought into consistent relations.
In the light of these principles the relations of protoplasm to the leading features of vegetable nutrition may be traced in brief outlines, as a prelude to the more highly differentiated processes of animal nutrition. The latest discoveries in physiology all tend to verify the conclusion that the simple chemical elements and binary compounds, which constitute the food of plants, are built up by successive steps of gradually increasing complexity into protoplasm, or living substance, the ultimate product of constructive metabolism, and that the energy expended in the work performed is stored in the form of potential energy as an essential element or condition of its constitution. From the instability of the exceedingly complex molecules of protoplasm, destructive metabolism immediately follows, and the proximate constituents