SECONDARY INCREASE IN THICKNESS OF STEMS AND ROOTS. 127 
new layers concentrically one on another. The mode in which the meristem itself 
is formed, and in which the secondary layers of tissue are produced from it, differs 
greatly according to the nature of the plant. The numerous special modes of 
increase in thickness may be classified under three types, viz. : — 
1. Type of the Arborescent Liliaceae. The innermost primary cortical layer 
produces a meristem, in which new closed fibro-vascular bundles continue to arise, 
which anastomose into a network, while the tissue between the bundles developes as 
secondary fundamental tissue. 
2. Type of normal Gymnosperms and Dicotyledons. The vascular bundles of 
the stem are open and arranged in a ring ; the generating tissue which lies between 
the phloem and the xylem of each bundle is also continued through the medullary 
rays, i. e. those parts of the fundamental tissue which lie between any two adjoining 
bundles. Thus arises a continuous ring of meristem, which, according to the old 
use of terms, and to distinguish it from the ring of meristem in the preceding type, 
is commonly called the Ca?)ibncm-ring. While in the preceding type new vascular 
bundles arise only in the ring of meristem, the meristem- or cambium-ring in 
this type crosses the primary vascular bundles, which have their phloem lying 
on the outer side, their xylem on the inner side of the cambium. The increase 
in thickness consists in new secondary xylem being continually formed out of the 
cambium-ring on its inner side, new phloem or secondary cortex on its outer 
side. 
3. The type of Roots (of Gymnosperms and Dicotyledons). In the axial fibro- 
vascular cylinder or plerome-bundle there lie, as has been mentioned, alternate 
groups of vessels (xylem) and phloem-bundles side by side; on the inner side of 
each of the latter arises a cambium-layer, which produces secondary xylem on its 
inner side, phloem on its outer side ; on the outer side of the primary groups of 
xylem meristem is also formed, which either produces only secondary funda- 
mental tissue, or combines with the cambium-layers already mentioned into a 
complete cambium-ring, out of which xylem is again formed on the inside, phloem 
on the outside. 
Further details may now be given regarding each of these three types, with 
an example ; the nomenclature of the more important deviations, especially for the 
second type, will be deferred till the end of this section. 
(i) The Type of Arborescent LiliacecE is represented in the genera Draccena, Aleiris 
[Calodracofi)^ Yucca, Aloe, Lomaiophyllu??i, and Beaucarnea ^ Specimens of the old 
stems of these plants are often found in botanical collections so decayed that within 
the thin layer of periderm the whole of the parenchymatous fundamental tissue has 
completely disappeared, while the fibro-vascular bundles are preserved entire. If 
one of these stems is split lengthwise, it is seen that completely isolated bundles 
run down the middle, as is the case in all Monocotyledons. Each bundle begins 
below at the periphery of the stem ; higher up, it bends towards the centre of 
the stem, and then again outwards, finally entering a leaf at its upper end. The 
^ A fourth type may be furnished by the mode of the increase in thickness in the primeval 
LycopodiacecE {vide supra) ; but scarcely anything certain is at present known about it. 
