L 



DICOTYLEDONS. 65 1 



that longitudinal divisions are formed (as in Fiscum, Helianthus annuus, &c.). The 

 masses of xylem and phloem formed by the activity of the cambium-ring are pene- 

 trated lengthwise in radial direction by secondary medullary rays consisting of hori- 

 zontal cells which in the wood are not always lignified, and in the secondary cortex 

 are generally soft and parenchymatous. In the one case they are called xylem- 

 rays, in the other phloem-rays, and always have the power of taking up assimilated 

 food-materials. In proportion as the cambium-ring increases in size, the number of 

 these rays increases ; and the later layers of wood are always traversed by a larger 

 number of rays. They are one or more layers of cells in thickness, and form thin 

 vertical plates wedge-shaped at their upper and lower edges, which have the appearance 

 in a longitudinal section of ribbon-like structures (the 'silver-grain'). In a tangential 

 section the fibro-vascular bundles which run through the length of the stem are seen to 

 form a network of elongated meshes, through which the rays pass (especially clearly seen 

 in decaying cabbage-stumps). The medullary rays, like the fibro-vascular bundles, are 

 added to by means of the cambium-ring outwards and inwards ; and as the ring increases 

 in thickness, it produces new rays between the old ones. 



[The following tabular account of the structure of the secondary wood (xylem) in 

 Dicotyledons is taken from De Bary (Vergleichende Anatomic) : — 



1. Wood consisting only of tracheides with bordered pits : — 



Wintereae [Drimys Winteri, lasmannla aromatica ; also Trochodendron ara- 

 lioides): (Conifers^). 



2. Wood consisting of vessels, tracheides, parenchyma, and intermediate cells 



{ersat^fasern^) : — 



a. With no intermediate cells ; Ilex aquifolium, Staphylea pinnata, Rosa 



canina, Cratagus monogyna, Pyrus communis^ Spircca opulifolia, Ca- 

 mellia, &C. 



b. With no parenchyma ; Porlieria. 



c. With both parenchyma and intermediate cells; Jasmlnum renjolutum, 



Kerr'ta, Potentilla fruticosa, Casuarina eqwsetifolia and torulosa, Aristo- 

 lochia Sipho, &c. 



3. Wood consisting of vessels, tracheides, fibres, parenchyma, and intermediate 



cells : — 



a. With no intermediate cells; fibres unseptate; e.g. Sambricus nigra 



and racefnosa, Acer platanoides, pseudoplatanus, and campestris. 



b. With both parenchyma and intermediate cells ; fibres unseptate ; 



Berberis vulgaris, Mahonia ; [Ephedra). 



c. With no intermediate cells ; fibres septate and unseptate ; Punica, 



Euonymus latifolius and europceus, Celastrus scandens, Fitis 'vinifera, 

 Fuchsia globosa, Centradenia grandifolia, Hedera Helix, &c. 



d. With all four kinds of cells ; MUhlenbeckia complexa, Ficus (?). 



4. Wood consisting of vessels, tracheides, fibres, parenchyma, and intermediate 



cells. This is the most common, and may be taken as the typical struc- 

 ture. — 



a. With no intermediate cells ; Sparmannia africana, Calycanthus, Rhani' 



nus cathartic a, Ribes rubrum, Quercus, Castanea, Carpinus sp., 



Amygdaleae, Melaleuca, Callistemon sp., &c. 



^ [According to Sanio, Taxiis haccata has v^^ood of this composition, but Hartig and Kraus 

 state that some parenchyma is also present ; this parenchyma is better developed in the other 

 Conifers.] 



2 [See &upra, p. T19. These cells were first distinguished by Sanio, who termed them 'ersatz- 

 fasem' (replacing- fibres), because they frequently take the place of the wood-parenchyma. The 

 term 'intermediate' used above refers to the fact that they are intermediate in form between 

 prosenchymatous and parenchymatous cells.] 



