MODIFICATIONS OF LEAVES AND STIPULES. 



411 



MODIFICATIONS OF LEAVES AND STIPULES FOR 

 SPECIAL PURPOSES. 



By Rev. Professor G. Henslow, M.A., F.L.S., V.M.H. 



[Lecture delivered October 27, 1908.] 



The Origin of Leaves and Stipules. — A leaf may be regarded as 

 consisting of three parts, any one or two of which may be absent — 

 viz. the petiole, or stalk, with or without stipules, and the lamina, or 

 blade. This may be "simple" — i.e. of one piece — or "compound," 

 being divided into separate "leaflets." ' Transitions between deeply lobed 

 simple leaves and compound ones are frequent. 



The anatomical source of leaves is deep-seated in the shoot, the 

 epidermis being continuous externally. The woody cylinder of the stem 



Fig. 77.— Leaf of Pansy, showing the 

 Midribs of the Stipules. 



is composed of a ring of separate fibro-vascular bundles ; one leaves its 

 position and passes upwards through the cortex to supply the midrib 

 of the external petiole and blade. One or more arise from the cylinder 

 on each side of the leaf — not necessarily adjacent to it — and enter the 

 sides of the petiole, often causing this to be channelled above, as may be 

 seen in members of the Umbelliferae, supplying strength, just as in the 

 " stays " of a Fox's umbrella frame. 



If stipules are present their midribs arise as fibro-vascular bundles 

 from the outermost ones of the lateral bundles entering the petiole of 

 the leaf, and never directly from the stem-cylinder of bundles (fig. 77). 

 The violet, dock, and hazel furnish good examples. 



If there be two and opposite leaves at the same node, then a fibrous 

 "stipular zone" is formed within the cortex connecting the petiolar 

 bundles. The midribs of the stipules are supplied by this girdle, 

 or zone. A transverse section of the node of a Galium illustrates this 

 very well (fig. 78), stipular cords (a, b, c,) entering the petiole. 



