OF LEAVES IN GENERAL. 145 



stalk (petiole) is much extehded ia the lower leaves Of many 

 plants, especially of those which grow in the shade or are 

 intermixed with other plants. Structurally the petiole is the 

 extension of the flbro-vascular and parenchymatous connec- 

 tion between the leaf and the stem ; and it generally forms 

 an articulation or joint^with the stem at its lower extremity ; 

 physiologically it is & support for the leaf, and it is longer or 

 shorter just as elongation or want of it places the leaf under 

 the best physiological conditions. 



189. — The leaf is, when first formed, destitute of fibro-vas- 

 cular bundles, and this is the permanent condition of the leaves 

 of Bryophytes, and the leaf-like portions of the Thallophytes. 

 In most higher plants, however, portions of the leaf tissue 

 early become differentiated into one or more fibro-vascular 

 bundles, which pass downward into the stem and unite 

 with the older bundles ; the upper parts of the bundles grow 

 with the leaf, and form lateral branches and branchlets, 

 giving rise to the complicated system of so-called veins so 

 often to be seen (especially in Dicotyledons). In many of 

 the smaller phyllome structures, as scales, bracts, etc., which 

 may be regarded as rudimentary leaver, there are no fibro- 

 vascular bundles, jnst as in the rudiments of actual leaves. 



190.— Venation. In mosses and other plants destitute of 

 fibro-vascular bundles, the veins, when present, are composed 

 of but slightly modified parenchyma ; in higher plants they 

 are composed of fibro-vascular bundles and, in the larger 

 veins, of one or more surrounding layers of modified paren- 

 chyma in addition. 'The disposition of the vcans in a leaf 

 depends largely upon its mode of growth. Usually several 

 veins form early ; if they grow from a common point, an 

 arrangement like that in the maple (radiate venation) is the 

 result ; if the veins grow from points on an axis, the various 

 modifications of the pinnate venation are produced, depend- 

 ing upon the amount of elongation of the axis. 



In many Monocotyledons the leaves continue to grow at 

 their bases ; their veins are, as a consequence, parallel with 

 the leaf axis ; in other Monocotyledons and most Dicoty- 

 ledons the veins originate on an extending axis, and pass 

 outward to or near to the margins. 



