654 PH A NBR OGAMS, 
a. Both the primary and the secondary bundles remain isolated ; they are not 
united by a closed cambium-ring, but anastomose with one another, as in Cucurbita, 
Nymphaeaceae, and Papa'ver. In the Gucurbitacese and Piperaceae the more internal 
bundles are arranged in a ring, but in the Nymphaeaceae they are arranged irregularly, 
so that the transverse section of the stem bears a greater or less resemblance to that 
of a Monocotyledon. 
b. The primary bundles lie in a ring on the transverse section, and are united by 
a cambium-ring ; the secondary bundles arise at -an early period in the pith and remain 
isolated and scattered on the transverse section ; they anastomose with one another and 
with the primary bundles in the nodes of the stem. Examples are furnished, according 
to Sanio, by Begoniaceae, Aralia, and some Umbelliferae. 
The Cell-forins of the phloem and xylem of Dicotyledons have already been described 
in general terms (see p. ii6 seq.). Only two peculiar phenomena need be mentioned 
here. In Cucurbitaceae, some Solanaceae, and Nerium (and in a certain sense also in 
Tecoma radicans'^), a phloem-tissue is found not only on the outside but also on the inside 
of the fibro-vascular bundles, which is developed with especial strength in Cucurbitaceae. 
' The isolated fibro-vascular bundles in the pith which are enclosed by the ring of wood 
sometimes show an abnormal arrangement of their phloem and xylem. Thus, according 
to Sanio, Aralia racemosa has, within the normal circle which grows by means of a 
■ cambium-layer, an endogenous circle of closed fibro-vascular bundles in which the xylem 
is peripheral and the phloem central as regards the stem. The isolated bundles in the 
pith of Phytolacca dioica on the other hand consist, according to Nägeli, on a transverse 
section, of a hollow woody cylinder which surrounds the phloem on all sides and is 
penetrated by xylem-rays. The isolated fibro-vascular bundles of the pith in the rachis 
of the inflorescence of Ricinus communis also consist of a thin axial bundle of phloem (?), 
surrounded by a sheath of cells (xylem ?) arranged in rays. 
A layer of collenchyma is very common in Dicotyledons beneath the epidermis of 
the internodes and leaf-stalk. 
The Classification of Dicotyledons ^ has now been carried out so completely that the 
smaller groups which are called Families ^, and which usually comprise genera very 
nearly related to one another, have been united into larger groups or orders ; so that 
at present only a few families remain unplaced. The greater number of the orders can 
also be again arranged into larger groups which are clearly connected by actual relation- 
ship. Systematists have not however up to the present time agreed as to how many of 
these cycles of affinity should be established, so as to make the primary division of the 
whole class of Dicotyledons in accord with the requirements of scientific classification. 
The grouping of all Dicotyledons into three sections, Apetalae, Gamopetalae, and Eleu- 
theropetalae, proposed by De Candolle and Endlicher*, is now abandoned by most, 
although still much in use for practical purposes. A. Braun ^ placed among the Eleu- 
theropetalae the greater number of plants previously classed among Apetalae ; and 
Haustein^ has now distributed among them the remainder, so that the whole class 
consists of only two sub-classes, Gamopetalae and Eleutheropetalae. This classifica- 
^ [A cambium-ring is formed internally to the primary bundles in the stem of this plant ; see 
Nägeli, Beiträge, I : also Sanio, Bot. Zeitg. 1864, p. 228.] 
^ [See note to p. 630.] 
^ Le Maout and Decaisne's Traite general de Botanique, descriptive et analytique, is strongly to 
be recommended for a study of the diagnosis of the families [translated by Mrs. Hooker ; London 
1873]- 
* Endlicher, Genera plantarum secundum ordines naturales disposita, Vindobonse, 1836-1840 ; 
and Enchiridion botanicum, Lipsise — Viennae, 1841. 
A. Braun, Uebersicht des natürlichen Systems, in Ascherson's Flora der Provinz Brandenburg, 
1864. 
Hanstein, Uebersicht des natürlichen Pflanzensystems, Bonn 1867. In the first edition of this 
