Exercise VII 



COLLEXCIIYMA CELLS 



I. Introduction. — The subepidermal region of many stems, peti- 

 oles and ribs of leaves is occupied by a more or less "simple" 

 or homogeneous tissue which is termed collenchyma. Frequently 

 this tissue occurs as a continuous hypodermal cylinder but in 

 some stems and in the petioles of leaves, distinct strands of col- 

 lenchyma may be present. An interesting example of the strand- 

 like arrangement of collenchyma is furnished by the familiar 

 "strings" present in the abaxial ribs of celery petioles (cf. Esau, 

 1936, pi. 2 and 6). 



In certain respects, collenchyma is remarkably similar to 

 cortical parenchyma. Indeed Hay ward (p. 22) regards collen- 

 chyma as a derived form of parenchyma, a viewpoint also ex- 

 pressed by De Bary (pp. 119-120) who stated that "it is then 

 to a great extent a matter of taste how far one Avill extend the 

 term Collenchyma." The parenchymatous character of collen- 

 chyma is shown by the fact that it is a tissue composed of living 

 cells, the protoplasts of which, like parenchyma, are able to 

 revert to a meristematic state. This feature is illustrated by the 

 origin of the phellogen or cork cambium in the outermost collen- 

 chyma cells in many stems. Furthermore, collenchyma. although 

 primarily a "mechanical tissue" in its function, presumably car- 

 ries on some photosynthesis as evidenced by the frequent occur- 

 rence of chloroplasts in its cells. The shape of collenchyma cells, 

 especially those adjacent to parenchyma elements in the cortex 

 or petiole, may be more or less "isodiametric." Typically, how- 

 ever, collenchyma cells are elongated, prismatic elements Avith 

 obtuse, pointed or oblique ends. 



The most definitive characteristic of collenchyma cells is 

 found in the irr< (juJar and often mnssii'e fhirkeninr/a of the cell 

 wall. These wall thickenings, which are deposited by the proto- 

 plast in the form of extensive longitudinal .strips, vary in their 



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