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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 Liliacese. 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, /. 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 Cambium-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 LiliacecB is represented in the genera Draccena, Aleiris 

 (^Calodracon), Yucca, Aloe, Lomatophyllum, 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. 



