112 THE ELEMENTS OF VEGETABLE HISTOLOGY 



Collenchymatic Tissues. — Collenchymatic tissue 

 may be found in herbaceous or green stems, fruits, seeds 

 and leaves. This element not only serves purposes 

 of support, but is also an assimilation and storage 

 tissue. In the green stems of annual plants little 

 time is available for the formation of woody tissues 

 in large amounts, nor are these necessary when one 

 considers the short life of such plants. Collenchy- 

 matic tissues are more quickly produced and can be 

 so arranged in the herbaceous stem that they serve 

 the purpose of support quite as well as woody tissue. 

 The walls of collenchyma cells are composed of cellu- 

 lose, but the original cell wall is reinforced by deposi- 

 tion of additional layers of cellulose, until a thick, 

 strong wall is built up. Collenchyma appears in the 

 form of polygonal cells with their walls especially 

 thickened at the angles (Plate 43). The wall is white 

 and of pearly luster. Striations and pores may be 

 present, and the latter may communicate with cor- 

 responding pores of an adjacent cell. There is but 

 little intercellular space between collenchyma cells; 

 therefore the cell cavities stand out clearly in a white 

 background of wall substance. One must not mistake 

 these cell cavities for cells, and careful search is often 

 necessary to establish the boundaries of the cell walls 

 surrounding each cavity. In herbaceous stems, par- 

 ticularly the square stems of the Labiatse or Mint 

 family, the collenchymatic tissue is placed at the 

 four angles of the stem, with perhaps a secondary 

 group between those at each angle. This arrangement 

 places the collenchyma at the points of greatest stress. 

 Collenchyma is usually associated with the fibrous 

 tissues in the midrib of the leaf. 



