264 PHYSIOLOGICAL DEVELOPMENT. 



and still more conclusive, are the cases in which the under 

 side of the leaf, being more exposed to light than the upper 

 side, usurps the character and function of the upper side. 

 If a common Flag be pulled to pieces, it will be seen that 

 what answers to the face in other leaves, forms merely the 

 inside of the sheath including the younger leaves, and is 

 obliterated higher up. The two surfaces of the blade answer 

 to the two under halves of a leaf that has been, as it were, 

 folded together lengthways, with the two halves of its upper 

 surface in contact. And here, in default of an upper surface, 

 the under surface acquires its character and discharges its 

 function. A like substitution occurs in Aristea corymbosa; 

 and there are some of the Orchids, as Lockhartia, which dis- 

 play it in a very obvious way. 



When joined with the foregoing evidence, the evidence 

 which another kind of substitution supplies is of great 

 weight. I refer to that which occurs in the Australian 

 Acacias, already instanced as throwing light on morpho- 

 logical changes. In these plants the leaves properly so called 

 are undeveloped, and the footstalks, flattened out into folia- 

 ceous shapes, acquire veins and midribs, and so far simulate 

 leaves as ordinarily to be taken for them: a fact in itself of 

 much physiological significance. But that which it concerns 

 us especially to note, is the absence of distinction between 

 the two faces of these phyllodes, as they are named, and 

 the cause of its absence. These transformed petioles do not 

 flatten themselves out horizontally, so as to acquire under 

 and upper sides, as most true leaves do; but they flatten 

 themselves out vertically: the result being that their two 

 sides are similarly circumstanced with respect to light and 

 other agencies; and there is consequently nothing to cause 

 their differentiation. And then we find an analogous case 

 where differential conditions arise, and where some differ- 

 entiation results. In Oxalis bupleurifolia, Fig. 66, there is a 

 similar flattening out of the petiole into a pseudo-leaf; but 

 in it the flattening takes place in the same plane as the leaf, 



