INTRODUCTION 63 



})ositioii according to several definable types of patterns when the 

 cells are examined in trans-section. In perhaps the commonest 

 type, the thickened areas of the wall are largely restricted to the 

 corners where the cells meet. Collenchyma of this type is termed 

 "angular." In certain plants, the thickenings are confined to 

 the tangential walls of the cells, resnlting in the so-called lamellar 

 collenchyma. Ordinarily, intercellnlar spaces are small or even 

 absent in collenchyma tissne but in certain Compositae the thick- 

 ened areas of the walls of the collenchyma cells border upon large 

 and prominent air-spaces. The term "tubular'" has been applied 

 to this type of collenchyma. These "types" of collenchyma, 

 however, are not rigidly distinct, because in some instances transi- 

 tions from one to the other may occur in successive radial portions 

 of the same zone of collenchyma. 



The structure and chemical composition of the wall-thicken- 

 ings in collenchyma cells have recently been studied by several 

 investigators. Anderson (1927), in his studies on the angular 

 collenchyma of tomato {Solanum lycopersicum), concluded that 

 the thickenings, which have a high content of water, consist of 

 alternating lamellae of cellulose and pectin. When viewed under 

 crossed Nicols, the walls appeared bi-refringent. Esau (1936) 

 found that the walls of collenchyma cells in celery petioles "are 

 chiefly of cellulose and contain a high percentage of water. ' ' She 

 interprets the wall thickenings as representing a special develop- 

 ment of the primary w^all. Simple pits are found in the walls 

 of collenchyma cells but are not necessarily restricted to either 

 the thin or thickened areas. 



The thorough study of Esau (1936) has shown that the 

 ontogeny of collenchyma tissue in celery petioles exhibits many 

 interesting and important features. In the mature petiole, the 

 collenchyma occurs in the form of distinct strands which corre- 

 spond in position to the abaxial ribs. The origin of these collen- 

 chyma bundles is traceable to the localized periclinal and anti- 

 clinal subdivision of ground meristem cells in the young petiole. 

 Procambial-like strands of somewhat elongated, thin walled cells 

 are thereby produced and it is from such cells that the adult 

 collenchyma eventually differentiates. At first the young collen- 



