MODIFICATIONS OF LEAVES AND STIPULES. 



413 



Occasionally the edges of the petiole unite and so form a funnel-like cap 

 over the bud, as in the plane (fig. 79) and mock-orange (Philadelphus) 

 (fig. 80) ; or the stem and petiole may both swell above the bud, making 

 a little cavity for the bud, as in Kalmia (fig. 81). 



Of course, the main use of the petiole is to extend the blade so as to 

 expose it to the light. To impart strength to it the main fibrous bundle 



Fig. 80. — Philadelphus ; (L) Leaf, 

 (a, a) Line of separation of Petiole. 

 (S) Stem. 



Fig. 81. — Section of Stem (S) and 

 Petiole (L) of Kalmia, showing 

 swellings (^4) which protect the 

 Bud. 



shows in a transverse section the form of an arc or section of a circle, 

 the lateral bundles being above the ends. Thus the groove is formed as 

 stated. Occasionally, as in sycamore, they are arranged in a complete 

 cylinder, as in a stem. In climbing plants as Clematis, Wistaria, Solarium 

 jasminoides (fig. 82), &c, as soon as the petiole or stem has secured a hold 

 to support the plant, the fibrous bundles increase considerably in size to 



Fig. 83. — Sections of Petioles of So- 

 lanum jasminoides (a) before, (b) 

 after securing hold on support. 



Fig. 82. — Twining Petiole of Solanum 

 jasminoides. 



resist the strain now put upon them as seen in Darwin's figures of Solanum 

 jasminoides (fig. 83, a, b). In extending the base of the petiole round 

 the stem additional strength is acquired to support a heavy blade, as in 

 Polygonum and the Umbelliferae ; while in palms the fibres, besides 

 sheathing at the base, cross in two bands embracing the stem, the weight 

 of many palm leaves being enormous. 



