Thus, growing protoplasm can assume only such shapes as allow it to remain in constant touch with the outer world. A cubical or spherical mass of cells could not grow by the formation of new layers of cells on the outside, for these layers would deprive the centrally placed masses of cells of their conditions of existence. Similarly, an extended membrane of cells or an epithelial layer cannot add indefinitely to its thickness, else would the cells furthest removed from the outside be injured in their relations to surrounding things. To satisfy its essential conditions, protoplasm can grow only with a proportionate extension of its external surfaces. This is secured by the cells becoming arranged in threads and membranes, and its result is that the threads by branching, and the membranes by folding, produce structures whose complexity increases with growth. This conception that the shape of growing organisms is in many respects the necessary consequence of the specific characters with which protoplasm is endowed, explains the great contrast between animals and plants in their general organisation. The contrast is the result of the difference between animal and plant metabolism, and between the ways in which animals and plants obtain their food. Plant cells elaborate protoplasm from the carbonic acid of the air, water, and easily diffusible solutions of salts, obtained from the sea or from the soil. For the chemical work of combining these, they require the active energy of sunlight. We can now see the chief requirements to which the constitution and arrangement of the cells in a multicellular plant must be adapted. Plant cells may become clothed in a thick membrane, as that would prove no hindrance to the passage of gases and easily diffusible salts; but they must be arranged so as to present the greatest possible surface to the surrounding media (_i.e._, to the soil and the water, the air and the sunlight) whence is drawn their supply of matter and force. The cells must turn a broad face to the outside; this they do by becoming arranged in branching rows, or in leaf-shaped flattened organs. That they may suck up water and salts from the soil, the cells are arranged as a highly branched system of roots, covered with delicate hairs, and penetrating the soil in every direction. To inhale the carbonic acid from the air, and to be subjected to the influence of sunlight, the aerial part of the plant stretches out its branches towards the light, and becomes folded into the flat leaves, the structure of which reveals a suitability for assimilation. Thus the whole architecture of a plant is superficial and visible; internal differentiation into organs and tissues either is wanting, or, compared with animals, is very scanty. It is only in the higher plants that the internal fibro-vascular tissues appear; these serve a double purpose: they act as channels along which the sap passes, so