THE LEAF. 51 



first leaf ( VorUatt of the Germans) on a branch in many Mono- 

 cotyledons is of a different form from the rest, and is found in the 

 angle between the branch and the stem from which it springs. In 

 Dicotyledons there are often two such leaves at the base of a 

 branch, right and left, and occasionally they are united into a 

 tube. They are sometimes formed after the other leaves according 

 to Hofmeister. 



In some families the true-leaf region is clothed with petioles 

 expanded into the form of laminaB ; these are called phyllodes (figs. 

 57 and 58), and in such cases the true laminar region is often 

 partially or entirely suppressed. 



The transition from the petiolar leaf-scale organs into perfect leaves 

 with sheathing petioles may be observed not only in bulbs, but in many 

 Grasses with creeping stems, which exhibit, at the junction of the leaf- 

 scale and true-leaf regions, sheaths surmounted by short green lancet- 

 shaped laminae, increasing in length in successive leaves. 



Stipules. When the petiole appears as a distinct leaf-stalk, it 

 is often accompanied by a pair of more or less distinct foliaceous 

 appendages at its base, 'called stipules. When these exist, the leaf 

 is called stipulate (fig. 63) ; when they are absent, exstipulate. 



The presence or absence of stipules is often a very constant character of 

 Natural Orders. The various forms of stipulate petioles form a kind of 

 transition to the petioles with sheathing bases. 



Petiole. The petiole is usually of semicylindrical form, with the 

 flat surface above ; not unf requently this upper surface is channelled 

 (canaliculate}, giving a more or less crescentic section ; in a few 

 instances, especially in the Aspen, it is laterally compressed. 

 Where it is cylindrical its structure is like that of a branch. 



The stalk-like petiole is either simple, when it supports a single 

 blade, or it is branched or compound, when the blade is composed 

 of a number of distinct leaflets ; the branches are sometimes 

 called partial petioles, and may even be articulated at their points 

 of origin from the primary petiole. 



Compound petioles supporting the leaflets of compound leaves are 

 known from branches, which at first sight they resemble, by arising 

 independently from the stem, by having buds in their axils, and 

 by the absence of any indication of a leaf immediately beneath 

 them. 



Phyllodes. The flattened or leaf-like petiole, called a phyllode, 

 resembles a lamina, but is known by standing edgewise on the 

 stein that is, with its flat faces parallel with the direction of the 



E2 



