64 COLLENCHYMA CELLS 



chyma cells are relatively small in diameter but as the rate of cell 

 division slows down, the cells expand and gradually acquire the 

 characteristic angular wall thickenings. The relative prominence 

 of intercellular air spaces in the mature collenchyma strand de- 

 pends in general upon the time of origin of the tissue. Air spaces 

 are only present when the divisions leading to collenchyma devel- 

 opment occur in loosely-arranged ground meristem cells. 



Functionally, collenchj-ma tissue provides considerable 

 strength as well as elasticity to young stems and to leaves. Ac- 

 cording to Esau (1936), the collenchyma of celery petioles 

 mechanically "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." 



II. Material for the Study of Collenchyma. — The clearly de- 

 fined angular collenchyma in the petiole of Datura stramonium 

 provides excellent material and its general structure will now be 

 described. (Note: If Datura is not available, the angular collen- 

 chyma in the cortex of the stems of Solanum lycopersicum, 

 Cucurhita, or Begonia, or the collenchyma strands in celery peti- 

 oles, may be used.) 



Obtain a trans-section of the petiole of Datura and after 

 mounting it either in water or a .1% solution of neutral red, 

 examine under low magnification. The epidermis possesses char- 

 acteristically thickened outer walls covered by a prominent cuti- 

 cle ; a protoplast and small scattered ehloroplasts should be vis- 

 ible in some of the epidermal cells. Note the })rominent multi- 

 cellular mihranched hairs and study carefully the structure of 

 their component cells. Beneath the epidermis are found six or 

 more "layers" of typical angular collenchyma cells, the irregu- 

 hirly thickened walls of which exhibit, when viewed in water, a 

 characteristic pearly-white lustre. Tn sections mounted in dilute 

 neutral red solution,^ the wvalls are brilliantly stained and their 

 relationships more readily investigated. Select a Ihin well-cut 

 area in the collenchyma and study llie cells under hiizh magnifica- 

 tion. The cell cavities have a more or less undulate outline which 

 is the i-esnlt of the alternation of thin and greatlv thickened 



1 Cf. Appendix, p. 142. 



