248 PHYSIOLOGICAL DEVELOPMENT. 



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 phyllodcs, 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 differen- 

 tiation 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, 

 so as to produce an under and an upper surface ; and here 

 the two surfaces of the pseudo-leaf are slightly unlike — in 

 contour if in nothing else. 



§ 275. We come now to such physiological differentiations 

 among the outer tissues of plants, as are displayed in tho 

 contrasts between foliar organs on the same axis, or on 

 different axes — contrasts between the seed-leaves and the 

 leaves subsequently formed, between submerged and aerial 

 leaves in certain aquatic plants, between leaves and bracts, 

 and between bracts and sepals. To deal even briefly with 

 these implies information which even a professed botanist 

 would have to increase by special inquiries, before attempting 

 interpretations. Here it must suffice to say something 



