THEIR STRUCTURE AND FORMS. 



109 



Being two-ranked and closely crowded, the outer ones at their 

 base fold over or bestride the inner (as shown in the sectional 

 diagram, Fig. 217), whence the name of equitant. Above, the 

 contiguous halves of the inner face conge nitally cohere, and so 

 produce the sword-shaped or linear vertical blade which is 

 characteristic of Iris (Fig. 216) and the Iris family. In most 

 there is a farther complication, of an excep- 

 tional kind, viz. the development backwards 

 of a portion of blade from the midrib, often 

 forming most of the upper part of such leaves, 

 which therefore may really be said to develop 

 in the vertical plane. 



215. Leaves with no distinction of Parts, *. e. 

 of blade and petiole. This is the case in Iris 

 (Fig. 216), Daffodil, the Onion, and perhaps 

 of most parallel-veined leaves of Endogens. 

 Those expanded in the horizontal plane may 

 however be regarded as sessile blades : those 

 which are not expanded, but filiform, or needle- 

 shaped (acicular), or awl-shaped (subulate), may 

 be regarded either as homologous with petioles, 

 or as unexpanded blades, which amounts nearly 



to the same thing where there is no trace of a petiole at base. 



Under this head may be ranked the leaves of Pines (Fig. 248) ; 



also both the subulate and the 



scale-shaped and adnate leaves 



of Arbor Vitae, Red Cedar (Juni- 



perus Virginiana) , and other trees < 



of the Cypress tribe. (Fig. 218.) 



216. Stipules serving for Blade. 

 Lathyrus Aphaca is a good in- 

 stance of this (Fig. 219) ; the 

 petiole becoming a tendril, the 

 leaflets which its relatives bear 

 being wholly wanting, the ample 

 foliaceous stipules assume the 

 appearance of leaves. In some 



other species of Lathyrus, and in the Pea, equalty large stipules 

 share with the pair or pairs of leaflets in the functions of foliage. 

 On morphological evidence, we judge that the singular leaves of 



FIG. 218. A twig of Arbor Vitae, with both awl-shaped and scale-shaped leaves. 



FIG. 219. Lathyrus Aphaca: portion of stem, bearing a single leaf, which consists 

 of a pair of foliaceous stipules, and a petiole in the form of a tendril; in its axil a 

 flower-stalk. 



