MORPHOLOGY 



35 



developed on the submerged leaves of Utricularia (Fig. 34). The 

 entrance to each bladder is fitted with a small valve which permits the 

 ingress but not the egress of small water-animals. While such leaves 

 display a progressive metamorphosis, in other instances the modifica- 

 tions are of the nature of a reduction. A metamorphosis of the whole 

 leaf lamina, or a part of it, into tendrils (leaf-tendrils) is a compara- 

 tively frequent occurrence, especially among the Papilionaceae. In the 

 adjoining figure of a Pea leaf (Fig. 35), the upper pair of leaflets have 

 become transformed into delicate tendrils which have the power of 



Fig. H.—XJtHculariamdgarls. A, Part of leaf with several bladders (x 2). B, Single pinnule of leaf 

 with bladder (X 6). C (after Goebel), Longitudinal section of a bladder (x 28) ; v, valve ; a, 

 wall of bladder ; J, cavity of bladder. 



twining about a support. In the case of the yellow Vetchling, Lathyrus 

 Aphaca (Fig. 36), the whole leaf is reduced to a tendril and the function 

 of leaf-blade is assumed by the stipules (n). A comparison between 

 these two forms is phylogenetically instructive, as it indicates the steps of 

 the gradually modifying processes which have resulted in the complete 

 reduction of the leaf lamina of Lathyrus. But, for still other reasons, 

 the last case deserves attention, as it shows clearly the morphological 

 distinction between leaf and stem tendrils, and emphasises the value of 

 comparative morphological investigation. 



In Lathyrus Aphaca the stipules assume the function of the 

 metamorphosed leaf laminae ; in other instances, as in the case of the 

 Australian Acacias (Fig. 48, 7, 8, 9), it is the leaf petioles which, 

 becoming flattened and leaf-like in appearance, supply the place of the 

 undeveloped leaf -blades. Such a metamorphosed petiole is called a 



