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. [Illustration: 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).] [Illustration: PLATE 52 _A._ Longitudinal section of the pith parenchyma of grindelia stem (_Grindelia 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 occurring 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. CHAPTER VI AERATING TISSUE