The larger amount of plant tissue is composed of =parenchyma= cells. These cells vary from square to oblong, or they may be irregular and branched. The end walls are square or blunt, and the wall is composed of cellulose, with the exception of the wood parenchyma, which has lignified walls. There are seven characteristic types of parenchyma cells: (1) cortical parenchyma, (2) pith parenchyma, (3) wood parenchyma, (4) leaf parenchyma, (5) aquatic plant parenchyma, (6) endosperm parenchyma, (7) phloem parenchyma. [Illustration: PLATE 49 _A._ Cross-section of black Indian hemp (_Apocynum cannabinum_, L.). 1. Longitudinal section of a latex tube. 2. Cross-section of latex tube. 3. Parenchyma. _B._ Cross-section of a part of black Indian hemp root. 4. Cross-section of a large latex tube. 5. Parenchyma.] [Illustration: PLATE 50 LATEX VESSELS 1. Radial-longitudinal section of dandelion root (_Taraxacum officinale_, Weber). 2. Cross-section of sanguinaria root (_Sanguinaria canadensis_, L.). 3. Cross-section of dandelion root.] Parenchyma cells, cortical, pith, aquatic plant, leaf, flower, and endosperm, conduct in all directions--upward, downward, and laterally. The direction of conduction depends upon the needs of the different cells forming the plant. The fluids pass from the cell with an abundance of cell sap to the cell with less cell sap. In this wall all cells are provided with food. Parenchyma cells conduct water absorbed by the roots and soluble carbohydrate material chiefly. The walls of all the different types of parenchyma cells are composed of cellulose with the exception of the wood parenchyma cells, the walls of which are lignified. The end walls of non-branched parenchyma cells and the cell terminations of branched cells are very blunt. 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.