CROSS-SECTION SWEET ALMOND SEED 1. Epidermis. 2. Hypoderm. 3. Vascular bundle. 4. Middle layer. 5. Inner epidermis. 6. Endosperm. 7. Outer layer of the embryo. 8. Inner layers of the embryo.] The =middle layers=. The cells forming the middle layers (Fig. 4) have thin, wavy, light-colored walls which are frequently compressed, and it is with much difficulty that their outlines are made out. The =inner epidermis=. The cells forming the inner epidermis are rectangular in form, and they contain reddish-brown cell contents (Plate 123, Fig. 5). ENDOSPERM The =endosperm=. The cells forming the endosperm are large, rectangular in outline, usually one layer thick, and they contain aleurone grains. EMBRYO The =embryo=. The cells forming the outer layer of the embryo are smaller than the inner layers, and they are immediately inward from the layer of endosperm cells (Plate 123, Fig. 7). The cells forming the greater part of the embryo are large, rounded, and they contain aleurone grains and fixed oil (Plate 123, Fig. 8). In white and black mustard are characteristic mucilage and palisade cells. In mix vomica, stropanthus, and St. Ignatius’s bean are characteristic hairs. In physostigma and kola are characteristic starch grains. In henbane, capsicum, stramonium, lobelia, and belladonna seeds are characteristic epidermal cells. In areca nut, colchicum, saw palmetto, and nux vomica are characteristic thick-walled, reserve cellulose cells. In cardamon seed are aggregate starch masses with irregular outlines. In bitter and sweet almond, linseed, pepo, and stropanthus are aleurone grains. In bitter and sweet almonds are stone cells. In linseed, quince seed, and in white and black mustard are epidermal cells with mucilaginous walls and contents, etc. CHAPTER X ARRANGEMENT OF VASCULAR BUNDLES Having familiarized ourselves with the different types of mechanical and conducting cells, we shall now consider the different ways in which these cells are associated to form the =vascular= and =fibro-vascular bundles=. The simplest form of the vascular bundle occurs in petals, floral bracts, and leaves. In these parts the vascular bundle is made up of conducting cells only. In the great majority of cases, however, the conducting cells are associated with mechanical cells to form the fibro-vascular bundle. The fibro-vascular bundle is made up of, first, the =phloem=, which consists of sieve tubes, companion cells, bast fibres, and parenchyma; secondly, of the =xylem=, composed of vessels and tracheids, wood fibres and wood parenchyma; thirdly, of medullary rays (restricted to certain types); and fourthly, of the bundle sheath (restricted to certain types). TYPES OF FIBRO-VASCULAR BUNDLES