LEAVES AND BUDS. 133 



165. Stipules. Very commonly wing-like outgrowths anee from 

 the base of the petiole, and frequently these simply assume the form 

 of lateral outgrowths of the leaf-base. The organs in question are known 

 as stipules, although they cannot always be sharply defined. The 

 stipules vary much in size, form, and colour. When they are quite 

 small, dry, pale, and membranous they are usually functionless, but 

 frequently they form large coloured protective bud-scales (Magnolia, 

 Fig, Beech), and fall when the leaves unfold. In the False Acacia 

 (Robinia) the stipules are metamorphosed into spines (Fig. 98), and in 

 Smilax they become tendrils. In Peas (Figs. 97, 98) they are large and 

 green, and take the place and function of the foliage-leaves, which are 

 more or less completely modified into tendrils. 



Stipules are often relatively large while the leaf is young, and in such 

 cases they serve as a protection to the young buds or young foliage, 

 frequently falling off as the leaf expands. When present upon adult 

 leaves, the stipules aid in the production of organic food if they are 

 green, and they may also help to lead away rain-water from the plant 

 so that it can dry rapidly, or to direct the water to those regions of the 

 soil where the absorptive organs (root-hairs) are most abundant. 



In the Rhubarb family (Polygonaceae) the two stipules join to form 

 a tube (ochrea) enclosing the stem for a short distance above its junction 

 with the leaf. The basal portion of a Grass leaf ensheathes the stem for 

 some distance, and just between the stem and the base of the free portion 

 of the leaf there is a small crescentic scale on the upper surface of the 

 latter. This scale is termed the ligule. 



166. Petiole. Note that while some leaves have a distinct stalk 

 (petiole), in others the lamina extends down to the stem of the plant. 

 The latter are called sessile leaves. 



The petiole is usually directly prolonged into the lamina as the mid- 

 rib, but in the Garden Nasturtium ( Tropaeolum) and in the Pennywort 

 (Cotyledon) the stalk is attached to the middle of the under-surface 

 (peltate leaf). A transition stage is shown by the leaves of the Water 

 Lily. The petiole may be winged, and it is usually cylindrical or 

 semi-cylindrical, being frequently flat or even grooved on the upper 

 surface. 



The petiole is a special secondary structure developed in order to 

 enable the lamina to expose itself to suitable illumination, and it is not 

 always present. It contains strands of vascular tissue, which are con- 

 tinued into the lamina as the branching veins of the leaf. There is 

 usually a large median vein, lying above a prominent midrib, although 

 in other cases a number of veins of equal size enter the leaf from the 

 petiole. In Garden Nasturtium ( Tropaeolum) and Clematis the petiole 

 has the power of coiling around supports, and hence it acts as a tendril 

 in spite of the presence of the lamina or blade. 



167. Veins. The veins are simply conducting bundles which run 

 outwards from the stem into each leaf, and undergo considerable 

 branching throughout the substance of the lamina. The larger veins 



