DICOTYLEDONS. 6 



13 



cotyledons in this mode of increase of their stems in thickness, they agree almost entirely 

 in this respect with Gymnosperms, except that in these latter there are no pitted vessels 

 in the secondary wood. In this respect however, according to von Mohl, Ephedra 

 indicates a transition to Dicotyledons. The organisation of Dicotyledons shows also 

 in some sense a higher stage of development in the greater varieties of the forms of 

 cells of which the xylem and phloem are composed. 



A remarkable deviation from these normal processes is exhibited by the Sapindaceae. 

 In some plants of this order the stem has the ordinary structure ; but in others a 

 transverse section shows several smaller woody cylinders of various sizes outside the 

 usual one and lying in the secondary cortex. Each of these increases in thickness, like 

 the normal ones, by a cambium-layer which surrounds it, Nageli supposes the cause 

 of this structure to be that the primary fibro-vascular bundles of the stem do not lie in 

 a circle on the transverse section, but in groups more towards the outside or inside. 

 When the connecting bands of cambium are formed in the fundamental tissue, the 

 isolated bundles become united on the transverse section, according to their grouping, 

 into one (as in Paullinia) or several {e.g. Serjania) closed rings. 



The cause of a large number of deviations of different kinds from the normal 

 structure of the stem in Dicotyledons which occur in various families, is the formation 

 of other cauline bundles of later origin in the stem besides the common bundles, either 

 within the primary pith or outside the ring in which the common bundles lie. We owe 

 to Nageli a more exact knowledge of these cases, and more especially to the very 

 exhaustive labours of Sanio, which form for the most part the basis, in addition to 

 my own observations, of the following short sketch, without going in detail into special 

 cases ^. I must refrain, in particular, from giving a detailed account of the behaviour of 

 Sanio's thickening-ring or of Nageli's meristem-ring, as this would involve considerable 

 prolixity. 



These phenomena may be classified into two groups, according as the secondary 

 (cauline) bundles originate within or without the circle of the primary (common) 

 bundles. Sanio calls the former the endogenous, the latter the exogenous mode of 

 origin. 



First Group. The secondary bundles are formed outside the primary bundles 

 (exogenous). 



a. The primary (common) bundles lie near the axis of the stem, and remain more or 

 less isolated, while the secondary (cauline) bundles are formed by a closed cambium-ring 

 external to the primary bundles, which continues to grow on the outside (originally a 

 'thickening-ring' in Sanio's sense). Examples are furnished hy Mirabilis, Amaranthus, 

 Jltriplex, Chenopodium album, and probably by all the Nyctagineae and Mesembryan- 

 themaceae. 



b. The primary (common) bundles lie in a ring on the transverse section and continue 

 their growth by means of a closed cambium-ring, which however soon disappears. A 

 new cambium-ring is then formed outside the one which has disappeared, and another 

 one again outside this one when it has in turn disappeared. Several circles of fibro- 

 vascular bundles are thus formed, continually increasing in number. In many Meni- 

 spermaceae {e.g. Cocculus), the new outer circle of vascular bundles together with its 

 cambium-ring is developed from a ring of meristem which lies in the primary cortex 

 and therefore outside the primary bast, — a phenomenon which is repeated in the cortex 

 as its growth proceeds (Nageli). In Phytolacca, on the other hand, and, according to 

 Eichler, also in Dilleniaceae, Wistaria, Bauhinia, Polygaleae {Securidaca and Comesperma), 

 Cissus, and Phytocrene, the successive circles of bundles originate in the secondary bast. 



Second Group. The secondary bundles arise early after the primary bundles further 

 inwards or nearer the axis of the stem (endogenous). 



^ [Oliver has collected the bibliography of the structure of the stem of Dicotyledons in the Nat. 

 Hist. Rev. 1862, pp. 298-329, and^i863, pp. 251-258.] 



