THE OUTER TISSUES OF PLANTS. 257 



In such laterally-growing kinds as are shown in Fig. 196&, 

 the honey-combed under surface and the smooth leathery 

 upper surface, have their contrasts related to contrasted con- 

 ditions; and in the adjacently-figured Agarics, and other 

 stalked genera, the pileus exhibits a parallel difference, ex- 

 plicable in a parallel way. But passing over Cryptogams it 

 must suffice if we examine more at length these traits as they 

 are displayed by Phasnogams. Let us first note the dis- 

 similarities between the outer tissues of stems and leaves. 



That these dissimilarities arose by degrees, as fast as the 

 units of which the phasnogamic axis is composed became 

 integrated, is a conclusion in harmony with the truth that in 

 every shoot of every plant, they are at first slight and become 

 gradually marked. Already, in briefly tracing the contrasts 

 between the outer and inner tissues of plants, some facts 

 have been named showing, by implication, how the cessation 

 of the leaf-function in axes is due to that change of condi- 

 tions entailed by the discharge of other functions. Here 

 we have to consider more closely facts of this class, together 

 with others immediately to the point. On pulling 



off from a stem of grass the successive sheaths of its leaves, 

 the more-inclosed parts of which are of a fainter green than 

 the outer parts, it will be found that the tubular axis even- 

 tually reached is of a still fainter green; but when the axis 

 eventually shoots up into a flowering stem, its exposed part 

 acquires as bright a green as the leaves. In other Mono- 

 cotyledons, the leaf-sheaths of which are successively burst 

 and exfoliated by the swelling axis, it may be observed that 

 where the dead sheaths do not much obstruct the light and 

 air, the surface of the axis underneath is full of chlorophyll. 

 Dendrobium is an example. But when the dead sheaths 

 accumulate into an opaque envelope, the chlorophyll is ab- 

 sent, and also, we may infer, the function which its presence 

 habitually implies. Carrying with us this evidence, we shall 

 recognize a like relation in Dicotyledons. While its outer 

 layer remains tolerably transparent, an exogenous stem or 

 63 



