The size of lenticels will vary according to the type of the lenticel. In studying sections more attention should be paid to the character of the cells forming the lenticels than to the size of the lenticel. On cross-section the intercellular spaces (Plate 58) are triangular, quadrangular, or irregular. The spaces between equal diameter parenchyma cells is triangular if three cells surround the space, and quadrangular if four cells surround the space, etc. These spaces are in direct contact with similar spaces that traverse the tissue at right angles to its long axis. The branched mesophyll cells of the leaf and aquatic plant parenchyma (Plate 59) are arranged around irregular cavities. In leaves and aquatic plants these spaces run parallel to the long axis of the organ. In each of the above cases the cavity is formed by the separation of the cell walls. There is still another type of irregular cavities which is formed by the dissolution or tearing apart of the cell walls. Such cavities are found in the stems and roots of many herbs. The pith cells in the stems of many herbs become torn apart during the growth of the stem, with the result that large irregular cavities are formed. These cavities are usually filled with circulatory air. In the stems of conium, cicuta, angelica, and other larger herbaceous stems the pith separates into layers. When a longitudinal section is made of such a stem it is seen to be composed of alternating air spaces and masses of pith parenchyma. The intercellular spaces are very large in leaves where enormous quantities of carbon dioxide are vitalized in photosynthesis. [Illustration: PLATE 57 CROSS-SECTION OF ELDER BARK (_Sambucus canadensis_, L.). 1. Periderm. 2. Lenticel. 3. Phellogen.] [Illustration: PLATE 58 INTERCELLULAR AIR SPACES _A._ Cross-section of uva-ursi leaf (_Arctostaphylos uva-ursi_, [L.] Spreng.). 1. Irregular intercellular air spaces. _B._ Cross-section of the cortical parenchyma of sarsaparilla root (_Smilax officinalis_, Kunth). 1, Triangular intercellular spaces; 2, Quadrangular intercellular air spaces; 3, Pentagular intercellular air spaces.] [Illustration: PLATE 59 IRREGULAR INTERCELLULAR AIR SPACES 1. Skunk-cabbage (_Symplocarpus fœtidus_, [L.] Nutt.) 2. Calamus rhizome (_Acorus calamus_, L.).] In the rhizome of calamus and other aquatic plants the intercellular spaces are very large. The cells of these plants are arranged in the form of branching chains of cells which thus provide for large intercellular spaces. The cells of the middle layer of flower petals, like the mesophyll of leaves, is loosely arranged owing to the peculiar branching form of the cells. Seeds and fruits contain, as a rule, few or no intercellular spaces. CHAPTER VII SYNTHETIC TISSUE