166 



OEGANOGRAPHY. 



termed biternate; or when the common petiole divides at its 

 apex into three secondary ones, and each of these again divides 

 into three others, each of which bears three leaflets, as in the 

 Yellow Fumitory {Corydalis tutea), and Epimedium, the leaf is 

 triternate or trijply-ternate {fig. 345) ; when such a leaf is still 

 further divided, it is said to be decompound. 

 Fig. 345. 



Fig. 345. Triternate leaf of Bane-berry {AcUbo). 

 5. PETIOLE OR LEAF-STALK. 



The petiole or leaf-stalk is that part which connects the blade 

 of the leaf with the stem {figs. 247, p and 248, p). It is fre- 

 quently absent, and the leaf is then said to be sessile ( figs. 259 

 and 260). It consists of one or more fibro-vascular bundles 

 {fig. 346, /f), surrounded by parenchyma {pc), and the whole 

 covered by epidermis, which contains but few or no stomata. The 

 fibro-vascular bundles vary in their nature in the leaves of the dif- 

 ferent classes of plants, being merely prolongations of those of the 

 three kinds of stem already fully described ; thus, in Dicotyledo- 

 nous Plants, the vascular bundles {fig. 250) consist of spiral, 

 pitted, and laticiferous A'essels, and wood-cells, or of the same 



