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.
CHAPTER VI
AERATING TISSUE
The aerating tissue of the plant performs a threefold function: first, it permits the exchange of gases during photosynthesis; secondly, it permits the entrance of oxygen and the exit of carbon dioxide during respiration; and, thirdly, it permits the exit of the excess of water absorbed by the plant.
The above functions are carried on by the stomata, the water-pores, the lenticels, and the intercellular spaces of the plant. The stoma functions as the chief channel for the passage of CO₂-laden air into the leaf and of oxygen-laden air from the leaf to the atmosphere. The stoma also functions as an organ of transpiration, since through the stoma a large part of the excess water of the plant passes off into the air.
WATER-PORES
In certain plants the primary epidermis is provided with openings resembling stomata, but unlike stomata the orifice remains open, and instead of being located on the upper or lower surface of the leaf, they are located on the margin of leaves immediately outward from the veins. Water is given off to the atmosphere from these openings. Such an opening is usually designated as a water-pore.