270 MORPHOLOGY. 



than those produced upon the axis above the surface. The former have 

 usually the form of scales or spines. 



3. Leaves which bear leaf-buds in their axils are generally more varied 

 in form than such (bracts) as bear flower-buds in their axils. 



4. The forms of leaves on one and the same axis are commonly of 

 similar kind, or pass continuously into each other, within the limits of 

 one definite series. Yet there are some remarkable exceptions to this, as 

 in some Aracece, and especially in the Cycadacece. In these plants two 

 forms of leaf occur regularly upon the same axis : in the Aracece very 

 short membranous sheaths alternate quite regularly with leaves having 

 sheath, petiole, and lamina ; in Cycadacece most of the leaves are mere 

 broad fleshy scales, which are placed spirally round the thick undeveloped 

 stem, but among these occur, at first isolated, in well-grown stems more 

 frequently, the great, handsomely pinnate or variously divided leaves, 

 which regularly continue the spiral, taking the place of those scales : the 

 sheathing portion of these leaves corresponds exactly to one of the 

 scales ; instead of a developed petiole and lamina, the scale bears only a 

 little slender process. Only through most superficial observation could 

 Link have asserted that the leaves spring from the axil of a scale.* 



IV. If we examine the cotyledon of most Monocotyledons we 

 find that, in its gradual development, it completely encloses the 

 terminal bud (plumula) ; indeed that the exceedingly delicate, soft 

 cells of the two borders of it become in part so firmly united, that 

 they may be regarded as grown together, only a little fissure, which 

 exists in all Monocotyledons, remaining. In germination the de- 

 veloping bud has not room to protrude through the little fissure, 

 so that it pushes the borders of it more or less forward, and then 

 these appear as a peculiar appendage on the middle of the coty- 

 ledon, as a membranous expansion of the border of the lower part 

 of the leaf, or as lobes on its base. Similar conditions also occur 

 frequently in the later leaves. In the Dicotyledons, a like con- 

 dition presents itself not unfrequently ; either the borders become 

 expanded like a membrane on the base of a petiole or stalk-like 

 leaf, or the emerging bud lifts up a longer or shorter membranous 

 sheath, or peculiar lobules are formed on the base of the petiole, 

 sometimes assuming the form of leaflets, and even connected with 

 the petiole by an articulation. In all cases, without exception, 

 they are, from the course of the development, parts of a leaf de- 

 veloped principally at its base, and in their essential nature, wholly 

 identical structures throughout all the Phanerogamia, though they 

 may vary most abundantly in their appearance. They have ac- 

 quired very different names, which have been created, partly merely 

 for particular families, partly solely for particular foliar organs. In 

 the Grasses these parts are called the ligule (ligula) : in other Mo- 

 nocotyledons, sometimes vagina stipularis, if large and rising free 

 from the lowest part of the leaf; vagina petiolaris, if small and 

 showing itself first higher up the leaf : in the Dicotyledons petiolus 

 alatus, stipules adnatce, if on the margins of the leaf-stalk ; ochrea, 



* Wiegmann's Archiv, 184J, vol. ii. p. 372. 



