TAXONOMY 
309 
Stems, and roots of secondary growth with open collateral fibro- 
vascular bundles, radially arranged about pith. 
Exogenous stems. 
Medullary rays present. 
Cambium. 
Fig. 172-. —Morphology of a typical dicotyledonous plant. A, leaf, pinnately- 
netted veined; B, portion of stem, showing concentric layers of wood; C, ground- 
plan of flower (the parts in 5’s); D, perspective of flower; E, longitudinal section 
of seed, showing dicotyledonous embryo. {Gager.) 
Roots developing secondary structure. 
Flowers tetra- or pentamerous (parts of each whorl, four or five or 
multiple thereof). 
Sub-class a.—Archichlamyde^: 
Those dicotyledonous plants in which the petals are distinct and 
separate from one another or are entirely wanting. That group of 
