LEAF STRUCTURE 189 



parenchyma is usually present in the form of green 

 masses consisting either of mesophyll cells or con- 

 solidations of these with palisade and epidermis. 

 Owing to the thickness of these fragments, details 

 of cellular structure are apt to be obscured. 



Fibro-vascular Tissues. — The fibro-vascular ele- 

 ments or veins of the leaf ramify in the mesophyll 

 layer and correspond both in development and struc- 

 ture to the bundles of the stem. In dicotyledonous 

 leaves the bundles are of the collateral type, while 

 in monocotyledons the concentric and closed collateral 

 arrangements may prevail. As in other parts of the 

 plant, the fibro-vascular bundles function as supporting 

 and conducting tissues. The work of transportation, 

 or conduction of crude materials and manufactured 

 nutrients to and from the leaf, requires a large and 

 widespread vascular system; therefore the conducting 

 elements of the bundles are more developed than are the 

 supporting elements. The fibro-vascular tissue in the 

 leaf stem, or petiole, is continued into the midvein in 

 pinnately veined leaves, or into the primary veins in 

 leaves of palmate venation. The midvein or primary 

 veins branch repeatedly and form innumerable small 

 veins, thereby connecting with every part of the leaf. 

 As the fibrous elements of the bundles are often 

 comparatively small, collenchymatic tissues may be 

 developed in the vicinity of the larger veins, thus 

 affording additional support. The fibro-vascular 

 bundles are often surrounded by a sheath which cor- 

 responds to a rudimentary endodermis and which 

 may function as a conducting element. The fibro- 

 vascular bundles of the petiole and the sheaths sur- 

 rounding them communicate either directly or indi- 

 rectly, with bundles in the parent stem. In powdered 



