THE LEAVES OF VASCULAR PLANTS. 23t 



ture of the leaf, and the variations of which it is 

 susceptible. 



As long as the fibres which spring from the stem form 

 a bundle but Httle or not at all spreading, and differing 

 from the state of the leaf properly so called, to this 

 bundle the name of Petiole is given : it is the same 

 organ which is commonly called the stalk of the leaf. 

 In contradistinction to the petiole, we call the Limb all 

 the part where the fibres are more or less diverging, and 

 where their spreading out is more or less perceptible.. 

 There are some leaves the expansion of which com- 

 mences at the same point where the fibres quit the 

 stem : these are said to be sessile, in contradistinction 

 to petiolated leaves, or those furnished with petioles. 

 We shall see hereafter that there are also leaves devoid 

 of the limb, and reduced to a simple petiole. 



As leaves are furnished with or devoid of the petiole^ 

 they may be articulated at their base with the stem or 

 branch which bears them, and then they are said to be 

 articulated with the stem ; — or the petiole, or the 

 limb when it is absent, may be united to the stem with- 

 out an articulation ; we say then that the leaves are 

 continuous with the stem. The first organization 

 principally occurs in leaves with ramifying nerves, and 

 petioles which are not sheathing ; the second, in leaves 

 with simple nerves, sheathing petioles, or an embracing 

 limb. We shall see, by and by, that this character is 

 connected in an important manner with the duration of 

 leaves. 



When the parts of a leaf are articulated with one 

 another, we give to them collectively the name of a 

 Compound Leaf, and reserve that of Simple Leaves 

 for those all the parts of which are continuous. The 

 partial limbs of compound leaves bear the name of 

 Leaflets. 



