Low. — On Haastia pulvinaris. 153 



wood fibres comprise a large portion of the xylem, while 

 tracheides also are numerous. 



The various forms of tissue in the wood are all pitted, 

 except the spiral vessels at the inner edge of the bundle. 

 The fibres are thick-walled, with tapering ends, and only 

 shghtly pitted. The tracheides are not so long, have thinner 

 walls, are more densely pitted. The vessels when mature are 

 large, and very densely pitted. 



The pith consists of large, circular cells closely packed 

 together, with slightly thickened, pitted walls. 



A stem still older (fig. VI.) shows a broader band of cork, 

 with the same radial chinks. The cortex remains the same, 

 with its resin-passages, but is afterwards disorganized by the 

 formation of successively deeper bands of cork, But the 

 sclerenchyma, which was o)-iginally near the resin-passage, 

 .now appears sunk deeply in the phloem parenchyma, and is 

 in large masses. 



The xylem has undergone the ordinary changes of woody 

 stems, and consists wholly of thick-walled elements. The 

 vessels, which are not very numerous, are strongly marked by 

 their wide cavities. Tracheides are numerous, but the great 

 mass of the wood is made up of wood fibres. At this stage 

 the cells of the medullary ray are thickened. 



Boot. 



The young root shows externally* a small-celled layer of 

 .cells, bearing root-hairs — the piliferous layer (fig. VI]I^). 

 Beneath it is a layer of large, polygonal cells, with their 

 radial walls slightly thickened — the exodermis. The cortex 

 is of flattened cells loosely packed, and forms a wide band of 

 tissue. The vascular cylinder is irregularly four-cornered. 

 There are, at first, four alternating bundles of xylem and of 

 phloem ; but at this stage a formation of secondary wood and 

 of secondary bast has taken place. The primary elements are 

 still visible, especially those of the wood. The primary xylem 

 consists of four masses of vessels, each occupying a corner of 

 the cylinder. The narrow vessels situated at the outside of 

 each mass are the spiral vessels, while the large pitted vessels 

 radiate towards the centre, and have rather thinner walls. 

 These do not quite meet at the centre of the cylinder, and 

 there is a considerable development of ground tissue, with 

 thin walls. Outside this central mass is a thin band of 

 secondary xylem, of thin-walled cells, of rectangular shape, 

 arranged in rows, developed especially opposite the primary 

 phloem bundles. The cambium-ring is fully formed, and is 

 forming the secondary phloem, irregular, thin- walled cells. 

 The primary phloem is still visible. 



In an older root the outer layers of cells become strongly 



