Section of the leaf of a Cycad, being one of the most ancient styles of leaf of which the structure is known. _a_, upper epidermis; _b_, upper layer of cells, with grains of chlorophyll; _c_, lower layer of cells, with chlorophyll; _d_, lower epidermis; _e_, stomata, or breathing-pores, with contractile cells for opening and closing.]
Foliage from the coal-formation, showing some of the forms of leaves instrumental in accumulating the carbon of our coal-beds, by their action on the atmosphere under the influence of sunlight.]
From the boundless variety of illustrations which the animal kingdom presents I may select one--the contrivances by means of which marine animals are enabled to float or balance themselves in the waters. The _Pearly Nautilus_ (see Fig. 15) is one of the most familiar, and also one of the most curious. Its coiled shell is divided by partitions into air-chambers so proportioned that the buoyancy of the air is sufficient to counterpoise in sea-water the weight of the animal. There are also contrivances by which the density of the contained air and of the body of the animal can be so modified as slightly to disturb this equilibrium, and to enable the creature to rise or sink in the waters. It would be tedious to describe, without adequate illustrations, all the machinery connected with these adjustments. It is sufficient for our purpose to know that they are provided in such a manner that the animal is practically exempted from the operation of the force of gravity. In the modern seas these provisions are enjoyed by only a few species of the genera _Nautilus_ and _Spirula_; but in former geological ages, more numerous, as well as larger and more complex, forms existed. Further, this contrivance is very old. We find in the _Orthoceratites_ and their allies of the earliest Silurian formations these arrangements in their full perfection, and in some forms[12] even more complex than in later types.
Section of the Pearly Nautilus and its shell, showing that the animal occupies only the outer chamber, the others being filled with air and acting as a float whose buoyancy can be modified by the action of the tube, or siphuncle, passing through the chambers.]