114 ELEMENTS OF PLANT ANATOMY. 
A few general exceptions from the so-called normal type 
may be mentioned here. In certain stems are found what is 
known as the bicollateral bundle. For example, Tecoma radicans 
has two rings of cambium cells, the outer one developing nor- 
mally, phloem outward and xylem inward. The inner one joins 
the xylem on its outer limit and develops only phloem, and that 
toward the centre. Another similar form occurs in Cucurbita, 
Nerium, and other stems, where the regular cambium of the 
circle of bundles at first develops phloem in both directions ; 
and after a time it ceases to produce phloem toward the centre, 
and begins to develop xylem in that direction. Such double 
rings of phloem are found in quite a number of plants. 
In certain other stems the normal circle of bundles is supple- 
mented by other circles, which originate either at the point of 
vegetation, as does the normal one, or from a secondary cambium 
which rises in the primary rind. In the first case, the rings 
consist generally of leaf-trace bundles, and they may he either 
within the normal circle or without it; that is, in the rind or in 
the pith. When in the pith, there may be one or more circles 
of the bundles, or as in the cotyledons they may lie scattered 
irregularly on the cross-section of the pith. In stems of Umbel- 
liferae, Orobanchaceae, Begoniaceae, there are stem bundles in 
the pith. Finally, in many swamp and water plants belonging 
to dicotyledons there is in the stem a central vascular bundle, 
which either arises from the union of several leaf-traces, or it 
may be considered a single stem bundle united to those of the 
leaves at the nodes. Examples of this class are Hippuris, My- 
riophyllum, etc. Such central bundles are also found in several 
monocotyledonous stems, as in Corallorrhiza and Najas. 
Stems of the Coniferae and Gnetaceae are similar in general 
structure to those of the dicotyledons. There is, however, a 
histological difference in the Conifer stems which does not appear 
in the primary growth; that is, the xylem of the first year’s 
erowth contains all the elements common to it, while in all the 
