DICOTYLEDONS. 
651 
that longitudinal divis'ons are formed (as in Fiscum, Belianthus minims, Sic). 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 Anatomie) : — 
1. Wood consisting only of tracheides with bordered pits : — 
Wintereae {Drimys Winteri^ Tasmannia aromatica; also Trochodendron ara- 
lioides): (Conifers^). 
2. Wood consisting of vessels, tracheides, parenchyma, and intermediate cells 
{ersai'z.fasern^) '. — 
a. With no intermediate cells ; Ilex aquifolium, Staphylea pinnata, Rosa 
canina, Cratcegus monogyna, Pyrus communis, Spircca opulifol'm, Ca- 
mellia, &c. 
b. With no parenchyma; Porlieria. 
c. With both parenchyma and intermediate cells ; Jasminum re'volutum, 
Kerria, Potentilla fruticosa, Casuarina equisetifolia and torulosa, Aristo- 
lochia Sip ho, &c. 
3. Wood consisting of vessels, tracheides, fibres, parenchyma, and intermediate 
cells : — 
a. With no intermediate cells; fibres unseptate ; e.g. Sambricus nigra 
and racemosa, 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 latifoUus and europceus, Celastrus scandens, Vitis 'vinifera, 
Fuchsia globosa, Centradenia grandifolia, Hedera Helix, &c. 
d. With all four kinds of cells ; M'uhknbeckia 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 qfricana, Calycanthus, Rham- 
nus cathartica, Ribes rubrum, Quercus, Castanea, Carpinus sp., 
Amygdaleae, Melaleuca, Callistemon sp., &c. 
[According to Sanio, Taxus baccata has w^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 supra, p. T19. These cells were first distinguished by Sanio, who termed them 'ersatz- 
fasern ' (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.] 
