Conducting Tissue. Cortical Parenchyma

Cortical parenchyma (Plate 51) differs greatly in size, thickness of the walls, and arrangement. A study of the longitudinal sections of different parts of medicinal plants reveals the fact that the cortical parenchyma cells form superimposed layers in which the end walls are either parallel, in which case the arrangement resembles that of several rows of boxes standing on end, or the end walls of the cells alternate with each other, in which case the arrangement is similar to that of the arrangement of the bricks in a building.

In certain plants the cortical parenchyma cells are long and narrow and rectangular in shape, while in other plants the cells, although still rectangular in outline, are very broad and approach the square form.

All typical cortical parenchyma cells have uniformly thickened non-pitted walls. In most barks the parenchyma cells beneath the bark are elongated tangentially, but are very narrow radially. The cells are always arranged around intercellular spaces, which vary from triangular, quadrangular, etc., according to the number of cells bordering the intercellular space.

Pith Parenchyma

Pith parenchyma (Plate 52) differs from cortical parenchyma cells chiefly in the character of the walls, which are usually thicker and always pitted.

Plate 51. Parenchyma Cells.

1. Longitudinal section of the cortical parenchyma of celandine root (Chelidonium majus, L.) 2. Cross-section of the cortical parenchyma of sarsaparilla root {Smilax officinalis, Kunth).

Plate 52. A. Longitudinal section of the pith parenchyma of grindelia stem {Grin-delta squarrosa, [Pursh] Dunal).

1. Cell cavity.

2. Cross-section of the porous end wall.

3. Surface view of the porous side wall.

B. Cross-section of the pith parenchyma of grindelia stem.

1. Cell cavity.

2. Porous walls.

3. Pitted end walls

Leaf Parenchyma

The parenchyma cells (Plate 109, Fig. 1) of leaves, of flower petals, and the parenchyma cells of some aquatic plants are branched; that is, each cell has more than two cell terminations. These cell terminations are frequently quite attenuated and usually very blunt. Such a cell structure provides for a greater amount of intercellular space and a maximum exposure of surface. This arrangement makes it possible for the parenchyma cells of the leaf to absorb more readily the enormous amount of carbon dioxide needed in the photosynthetic process.

Aquatic Plant Parenchyma

The parenchyma of aquatic plants (Plate 59) has large intercellular spaces formed by the chains of cells.

Wood Parenchyma

Wood parenchyma (Plate 105, Fig. 3) cells are the narrowest parenchyma cells occuring in the plant. Their walls are always lignified and strongly pitted, and in some cases the end walls common to two cells are obliquely placed.

Phloem Parenchyma

Phloem parenchyma (Plate 100, Fig. 8) cells are usually associated with sieve cells. They are very long, narrow, and have thin, non-pitted walls. The thinness of the walls undoubtedly enables the cells to conduct diffusible food substance more quickly than the cortical parenchyma cells.

Palisade Parenchyma

Palisade parenchyma of leaves is of the typical parenchyma shape and the end walls are placed nearly on a plane, even when more than one layer is present. The cells are very small, however, and the walls are very thin and non-pitted.