§ 273. We will leave now these contrasts between the free surfaces of plants and their attached or imbedded surfaces, and turn our attention to the secondary contrasts existing between different parts of their free surfaces. Were a full statement of the evidence practicable, it would be proper here to dwell on that which is furnished by the inferior classes. It might be pointed out in detail that where, as among the _Algæ_, the free surfaces are not dissimilarly conditioned, there is no systematic differentiation of them--that the frond of an _Ulva_, the ribbon-shaped divisions of a _Laminaria_, and the dichotomous expansions of the _Fuci_ which clothe the rocks between tide-marks, are alike on both sides; because, swayed about in all directions as they are by the waves and tides, their sides are equally affected. Conversely, from the _Fungi_ might be drawn abundant proof that even among Thallophytes, unlikenesses arise between different parts of the free surfaces when their circumstances are unlike. In such laterally-growing kinds as are shown in Fig. 196_b_, the honey-combed under surface and the smooth leathery upper surface, have their contrasts related to contrasted conditions; and in the adjacently-figured Agarics, and other stalked genera, the pileus exhibits a parallel difference, explicable in a parallel way. But passing over Cryptogams it must suffice if we examine more at length these traits as they are displayed by Phænogams. Let us first note the dissimilarities between the outer tissues of stems and leaves.