12.] CHAPTER II. - SPECIAL MORPHOLOGY OF THE MEMBERS. 45 



clades, are flattened and leaf-like as in Ruscus (Liliacene), Phyllan- 

 thus (Euphorbiaceae), Miihlenbeckia (Polygonaceoe), Carmichaelia 

 (Papilionacece), Phyllocladus (Coiiiferee), and are either isobi- 

 laterally or dorsiventrally symmetrical. The phylloclades fre- 

 quently bear flowers but not always in the same position. Thus, 

 in Ruscus androgynwB the flowers are borne on the margin of the 

 phylloclade ; in Ruscus aculeatus and R. Hypoglossum, they are 

 borne on the upper surface of the phylloclade ; and in R. 

 Hypophyllum, on the under surface. 



Leaf -like branches are also formed in Asparagus ; they are not 

 flattened, but are small and acicular ; something of the same kind 

 also occurs in Equisetum. 



12. The Leaf. All leaves, except the seed-leaves or coty- 

 ledons, are developed exogenously as lateral outgrowths upon the 

 growing-point of a stem : they are 

 developed, as a rule, in progressive 

 (acropetal or basipetal) succession, 

 though irregularity in this respect 

 sometimes occurs in the case of floral 

 leaves. 



The leaf is developed either from 

 the superficial layer of cells only, or 

 from this and other deeper layers, 

 and soon appears as a lateral pro- 

 tuberance on the growing-point. At 

 this stage it is undifferentiated, and FIG. 26. Phylloclade of Ruscus 

 may be termed Cordial. In some !STS2-i > h 

 plants with very simple leaves (e.g. the phylloclade p is developed ; d 



Chara) the developing leaf undergoes leaf of M the . *W}* beari s 



' . flowers 61 in its axil. 



no further change beyond increase in 



size ; but in most plants the leaf undergoes differentiation or 

 segmentation along its longitudinal axis or phyllopodium. In the 

 most complete case, the phyllopodium is differentiated into three 

 regions : a basal portion, the leaf-base or hypopodium ; an apical 

 portion, the epipodium; and an intermediate portion, the meso- 

 podinm, leaf-stalk, or petiole] but the last-named portion is fre- 

 quently absent. Most commonly the leaf assumes a flattened form 

 in consequence of the development of a relatively thin mem- 

 branous win</ along one or other of these regions in the lateral 

 plane: the epipodium is typically winged, and then constitutes 

 what is known as the blade or lamina of the leaf ; the mesopodium 

 is rarely winged, the hypopodium more frequently so. 



