SECT. 1 



MORPHOLOGY 



45 



tendrils (leaf-tendrils) is of comparatively frequent occurrence, 

 especially among the Papilionaceae. In the leaf of the Pea (Fig. 48), 

 the leaflets of the upper pairs have become transformed into delicate 

 tendrils which have the power of twining about a support. In the 

 case of the yellow Vetchling, Lathyrus Aphaca (Fig. 49), the whole 

 leaf is reduced to a tendril and the function of the leaf-blade is 

 assumed by the stipules {ii). A comparison of these two forms 

 is phylogenetically instructive, as it indicates the steps of the gradu- 

 ally modifying processes which have resulted in the complete reduction 

 of the leaf lamina of Lathyrus. The comparison of the two preced- 

 ing cases with Am2)eloi)sis (Fig. 29) will make the distinction between 



Fig. 4S.— Portion of stem and leaf of the common Pea, 

 Pisum sativum, s, Stem ; n, stipules ; b, leaflets of the 

 compound leaf ; r, leaflets modified as tendrils ; a, floral 

 shoot. (A nat. size.) 



Fig. 49. — Lathyrus Aphaca. 

 s, Stem ; n, stipules ; 

 • 6, leaf tendril, (i nat. 

 size.) 



these tendrils and stem-tendrils clear, and indicate the value of 

 comparative morphological investigation. 



In Lathyrus A]\)liaca the stipules assume the function of the meta- 

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

 Australian species of Acacia (Fig. 174, 7, 8, 9, Fig. 185), 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 phyllode, and, except that it is expanded 

 perpendicularly, closely resembles a phylloclade. From the latter, 

 however, it is morphologically different, for the one represents a 

 metamorphosed petiole, the other a metamorphosed shoot. In 

 accordance with this distinction phyllodes do not, like phylloclades, 

 spring from the axils of leaves. Just as stems become modified into 

 thorns (Fig. 30), by a similar metamorphosis leaves may be converted 

 into LEAF THORNS. Whole leaves on the main axis of the Barberry 

 (Berberis vulgaris) become thus transformed into thorns, either single or 



