240 PHYSIOLOGICAL DEVELOPMENT. 



length these traits as they are displayed by Phaenogams. Let 

 us first note the dissimilarities between the outer tissues of 

 stems and leaves. 



That these dissimilarities arose by degrees, as fast as tlie 

 units of which the phaenogamic 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 cessa- 

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

 conditions 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 



ofi* 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 Endogens, 

 the leaf-sheaths of which are successively burst and exfo- 

 liated 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. 

 Dendrohium is an example. But when the dead sheaths 

 accumulate into an opaque envelope, the chlorophyll dis- 

 appears, and also, we may infer, the function its presence 

 habitually implies. Carrying with us this evidence, we shall 

 recognize a like relation in Exogens. While its outer layer 

 remains tolerably transparent, an exogenous stem or branch 

 continues to show, by the formation of chlorophyll, that it 

 shares in the duties of the leaves ; but in proportion as a 

 bark which the light cannot penetrate is produced by the 

 adherent flakes of dead skin, or by the actual deposit of a 

 protective substance, the diflPerentiation of duties becomes 

 more decided. Cactuses and Euphorbias supply 



