34 CHANGES IN PARTS OF LEAVES. 
leaves when fully cultivated; and is considered to be 
a branch suppressed from some cause. 
OF THE CHANGES WHICH THE DIFFERENT PARTS 
OF THE LEAF UNDERGO. 
387. It has been stated (par. 7.) that a leaf is usually 
composed of a petiole, or leaf-stalk, and that in the 
leaves of some plants there are appendages called 
stipules (par. 9.); these different parts are variously 
modified or changed in some plants; and of these the 
following are examples. 
The petiole in the generality of plants will be found 
to be jointed to the stem of the plant, as in the Oak, 
Elm, Lime, Plane, Lilac, &c. so that in falling off, a 
mark or scar is left on that part of the stem to which 
it has been fixed. The petiole is absent in the leaves 
of some plants; as in the Pink, Sweet William, Snap- 
dragon, &c. (pl. 2. f. 8.); but in others, the petiole 
becomes widened where it reaches the stem, and instead 
of being fixed there, it surrounds it and passes down it 
like a sheath, and ends at a joint or swelling on the 
stem. This kind of petiole is seen in any Grass, Oat, 
Wheat, (pl. 12. f. 5. p. f. 6.), Barley, &c. and is called 
a sheathing petiole. 
38. In the Compound Leaf of the Pea, and Vetch 
(pl. 12. £. 7.), the upper leaflets, or those towards the 
apex of the leaf, will be found changed into what are 
termed tendrils; by these the plant is enabled to lay 
hold of neighbouring objects, and support itself in the 
air, In the Furze (pl. 12. f. 8.), the leaflets are 
changed into thorn-like parts, which give the plant a 
