THE LEAF 61 



the leaves of Dicotyledons. In Monocotyledons the main veins 

 run parallel to one another, but still they are connected laterally by 

 transverse branches. Thus the vascular system, of which the leading 

 function is conduction, is very effectively placed for carrying out its 

 purpose. For it is connected below with the conducting system of 

 the stem, and it spreads throughout the blade, and reaches to its 

 extreme tip and margin. 



A superficial examination gives only a very imperfect idea of the 

 structure of this important part of the plant. Transverse sections 



o.6t ax. 



coil 



FIG. 44. 



Transverse section of the midrib of the leaf of Aspen (Populus tremula), extending 

 to the thinner expanse of the blade right and left. ue= upper epidermis. le= lower 

 epidermis. pp= palisade parenchyma. sp= spongy parenchyma. co=collen- 

 chyma. #y=xylem. phl= phloem. s=stoma. Note numerous crystals of 

 calcium oxalate. adax= adaxial or upper, 6a# = abaxial or lower leaf surface. 

 ' (x75-) 



through the thin lamina so as to traverse one of the ribs trans- 

 versely, reveal the characteristic structure upon which the functional 

 activity of the foliage leaf depends (Fig. 44). The upper (ad-axial) face 

 is easily distinguished from the lower (ab-axial) by the fact that the 

 larger veins project strongly from the latter surface. Where the vein 

 is traversed it shows on a reduced scale the same construction as the 

 petiole, and like it the ribs are often strengthened by collenchymatous 

 tissue, as is seen in the Aspen. Here again the xylem of the vascular 

 bundle is directed towards the upper (ad-axial) face, and the phloem 

 towards the lower (ab-axial). The tissues which compose the thinner 

 expanse of the lamina bear definite relations to these opposed surfaces, 

 and to the incidence of light upon that which faces upwards. They 

 will be described, starting from the upper (ad-axial) face, as they might 



