468 



THE STRUCTURE OF ECONOMIC PLANTS 



More recently Esau (7), who attacked the problem by com- 

 paring the relative mechanical properties of the collenchyma and 

 vascular tissues, concluded that "Mechanically the collenchyma 





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" ' I III iri"'"ri ' 



;.A. 



Fig. 2.39. Upper, transection through mature petiole showing distribution of collenchyma 

 and vascular bundles. Schizogenous lacunae have developed in parenchyma. Lower, 

 transection of somewhat less mature petiole showing same features except for absence of 

 lacunae. In both figures, the abaxial surface is upward. (Photomicrographs by Esau, 

 Hilgardia.^ . 



is much stronger than the vascular tissue. The breaking load of 

 collenchyma may be two to four times that of the entire vascular 

 bundles or the bundle cap." 



In transection, the crescentic petiole is broadly expanded at its 

 base and more subcylindrical above with an adaxial groove extend- 



