DICOTYLEDONS, ;-«r 



a like volume formed in the spring \ While Dicotyledons differ- so widely from Mono- 

 cotyledons in the 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 v. 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 Sapindacea^. 

 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. 

 \\'hen 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 PauUiniaj 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 Njigeli 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 '*. 



The 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 bundles lie near the axis of the stem, and remain more or less iso- 

 lated, while the secondary bundles belong to a closed cambium-ring which continues to 

 grow on the outside (originally a 'thickening-ring' in Sanio's sense). Examples are 

 furnished by Mirabilis, Amaranthus, Phytolacca, and Atriplex, 



b. The primary 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 



' The cause of this difference is not yet knosvn ; but T suppose that it depends simply on the 

 difference in pressure to which the cambium and the wood are subjected from the surrounding cortex. 

 This pressure is less in the spring, and constantly increases till the autumn. I have no direct 

 measurements of this, but conckide it from the fact that the longitudinal fissures in the bark 

 become wider in February and March, as may be clearly seen in the oak, maple, poplar, walnut, &c. 

 I cannot here explain the cause of this ; but in any case the bark, the longitudinal fissures of which 

 have become wider in winter, must exert less pressure on the cambium in the spring, and the cells of 

 the wood must therefore be able to extend more easily in a radial direction. The pressure which 

 the bark exerts on the cambium must continually increase by the thickening of the ring of wood 

 internally and the drying up of the bark in summer externally, and must affect the radial growth of 

 the young cells of the autumnal wood. Further investigations which I am proposing to make will 

 determine whether my theory is correct. This hypothesis, which I brought forward in the first 

 edition, has recently been fully confirmed by the researches of H. de Vries. (See Flora 1872, no. 16, 

 and sect. 15 of Book III of this work). 



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

 Hist. Rev. i>:62, pp. 298-329, and i86,^ pp. 251-258.— Ed.] 



