•66 



BOTAXY 



PART I 



side of the petiole. In many species of Galium, where the stipules 

 resemble leaf-blades, the leaf-whorls appear to be composed of six 

 members, but consist actually of but two leaves with their four 

 stipules, which may be easily distinguished b}'' the absence of any 

 buds in their axils. In other species of the same genus {Galium 

 ermiatum and palustre) there are only four members iji the whorls, as 

 each two adjoining stipules become united. In many cases, as in the 



Rose and the Clover, the 

 stipules have the form of ap- 

 pendages to the enlarged leaf- 

 base. Sometimes both stip- 

 ules are united into a single 

 one, which then appears to 

 have an axillary origin ; or 

 the stipules may completely 

 encircle the stem, and thus 

 form a sheath (ochrea) about 

 the younger undeveloped 

 leaves. This sheath - like 

 fusion of the sti})ules may be 

 easily observed on the India- 

 rubber tree (Ficus elastica), 

 now so commonly grown as 

 a decorative plant. In this 

 case the stipular sheath is 

 burst by the unfolding of 

 each new leaf and pushed 

 upwards. In the Polygon- 

 aceae the stipular covering is 

 similarly torn apart by the 

 developing leaves, but re- 

 mains on the stem in the form 

 of a membranous sheath. 

 Heterophylly. — Some 

 plants are characterised by the development of different forms of foliage 

 leaves. Such a condition is known as heteroi^hylly. Thus the earlier 

 leaves of Eucalyphis globulus are sessile and oval, while those subse- 

 quently formed are stalked and sickle-shaped. It is common to find 

 that the leaves of the seedling plant have a simple form and are only 

 gradually succeeded by those of more complicated shape. In a similar 

 way Goebel {^^) explains the relation of the ribbon-shaped or finely 

 divided submerged leaves of amphibious plants to the aerial leaves with 

 stalks and entire blade which follow. Tlie former he regards as youth 

 forms, which are retained in the water, and not as a special adaptation 

 to the surrounding medium. In Fig. 37 the two types of leaf are 

 shown in the A\'ater Crowfoot (Rammculus aquafilis). A number of 



Fio. 37. — Ranunculus aquatilis. ub, Submerged leaves 

 s6, floating leaves ; 6, flower ; /, fruit. (Reduced.) 



