4So 



THE STRUCTURE OF ECONOMIC PLANTS 



tion was also noted in the breaking load when young collenchym- 

 atous strands were compared with older ones, the marked increase 

 in toughness with increasing age of the tissue being correlated 

 with increase in thickness of the cell walls. 



The Lamina. — The blade of the much incised and compounded 

 leaflet is relatively thin. The epidermal cells, as seen in surface 



— ph d 



Fig. 146. A, transection of portion of lamina; B, transection through midrib of leaflet: 

 col, collenchyma; d, oil duct; pal, palisade cells; ph d, phloem oil duct; tpo, spongy 

 parenchyma. 



view, are of two shapes; those subtending the collenchymatous 

 strands above the veins being rectangular with the elongated axis 

 parallel to the vein, while those which overlie the chlorenchyma 

 of the mesophyll have sinuous walls. The stomata occurring in 

 this type of epidermis are bounded by two guard cells containing 

 chloroplasts, and are surrounded by accessory cells as described 

 for the petiole. The stomata occur on both surfaces, but are 

 twice as numerous on the under one. Nestel (i6) reports a count 



