In the closed collateral bundle the phloem and xylem are not separated by a cambium layer, and in many cases the bundle is surrounded by thick, angled walled fibres, as in palm stem. The term closed bundle refers to the fact that there is no cambium between the xylem and phloem, therefore the bundle is “closed” to further growth, and not to the fact that it is frequently surrounded by fibres which prevent further growth. In podophyllum stem (Plate 126) the xylem portion of the bundle faces the centre of the stem and the phloem portion of the bundle faces the epidermis. The xylem and phloem are separated by a cambium layer, and both are surrounded by thick-walled angled fibres which are the chief mechanical cells of the stem. This bundle is, in fact, mechanically closed, but not physiologically because a cambium is present. [Illustration: PLATE 126 CROSS-SECTION OF A CLOSED COLLATERAL BUNDLE OF MANDRAKE STEM (_Podophyllum peltatum_, L.) 1. Vessels. 2. Sieve cells. 3. Cambium. 4. Fibres. 5. Parenchyma. 6. Intercellular space.] [Illustration: PLATE 127 BI-COLLATERAL BUNDLE OF PUMPKIN STEM (_Curcurbita pepo_, L.) 1. Vessels. 2. Sieve tubes.] BI-COLLATERAL VASCULAR BUNDLES In the bi-collateral vascular bundle (Plate 127) the xylem is in between two groups of phloem--namely, an inner group and an outer group. In pumpkin stem a bundle occurs in each angle of the stem. The entire bundle is surrounded by parenchyma cells. In an individual bundle the xylem consists of large circular vessels and a phloem containing large sieve cells, many of which show the yellow porous sieve plates. OPEN COLLATERAL VASCULAR BUNDLES In the open collateral bundle (Plate 100) the xylem and phloem are separated by the cambium layer, which, through its divisions, causes the stem to increase in thickness each year. This type of bundle is characteristic of the stems and roots of dicotyledonous plants. The bi-collateral bundle occurs in many leaves. The xylem in such cases is central, the phloem strands occupying upper and lower peripheral positions. INDEX Abbé condenser, illustration, 11 Absorption tissue, introduction, 121 tissue of leaves, 125 Aerating tissue, introduction, 151 Annular vessels, illustration of, 129 Bark, of white pine powdered, description of, 250 of white pine powdered, illustration of, 251 unrossed white pine, cross-section, illustration of, 249 Barks, description of, 248 diagnostic structures of, 253 structural variations of, 252 Base sledge microtome, 35 sledge microtome, illustration, 35