DICOTYLEDONS. 653 
cotyledons in this mode of increase of their stems in thickness, they agree ahnost 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. Nägeli 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 Paullinid) 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 Nägeli 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 Nägeli'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 by Mirabilis, Amaranthus, 
Atriplex^ 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 (Nägeli). 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.] 
