106 HISTOLOGY OF MEDICINAL PLANTS 



wood fibres frequently contain storage starch. The wood fibres 

 of logwood and red saunders contain coloring substances, which 

 are partially in the cell cavity and partially in the cell wall. 

 The walls of wood are composed largely of lignin. 



COLLENCHYMA CELLS 



Collenchyma cells form the principal medicinal tissue of 

 stems of herbs, petioles of leaves, etc. In certain herbs the 

 collenchyma forms several of the outer layers of the cortex of 

 the stem. In motherwort, horehound, and in catnip the col- 

 lenchyma cells occur chiefly at the angles of the stem. In 

 motherwort (Plate 29, Fig. B) there are twelve bundles, one 

 large bundle at each of the four angles, and two small bundles, 

 one on either side of the large bundle. In catnip (Plate 29, 

 Fig. A) there are four large masses, one at each angle of 

 the stem. 



Collenchyma cells differ from parenchyma cells in a number 

 of ways: first, the cell cavity is smaller; secondly, the walls 

 are thicker, the greater amount of thickening being at the angles 

 of the cells that is, the part of the cell wall which is opposite 

 the usual intercellular space of parenchyma cells, while the wall 

 common to two adjoining cells usually remains unthickened. 

 In horehound stem (Plate 30, Fig. 2) the thickening is so great 

 at the angles that no intercellular space remains. In the side 

 column of motherwort stem (Plate 30, Fig. i) the thickening 

 between the cells has taken place to such an extent that the 

 cell cavities become greatly separated and arranged in parallel 

 concentric rows. 



The collenchyma of the outer angle of motherwort stem 

 (Plate 30, Fig. 3) is greatly thickened at the angles. There 

 are no intercellular spaces between the cells, and cell cavity 

 is usually angled in outline instead of circular, as in the cells 

 of horehound. In certain plants intercellular spaces occur be- 

 tween the cells, and the walls are striated instead of being non- 

 striated, as in the stems of horehound, motherwort, and 

 catnip. 



Collenchyma cells retain their living contents at maturity. 

 Many collenchyma cells, particularly of the outer layers of 



