DICOTYLEDONS 205 



the light and air. The stem normally has bark on the 

 outside. Within the bark, reading from circumference 

 to centre, there are woody fibre, spiral vessels, cellular 

 tissue, and the central pith. The stem grows in diameter 

 by deposits formed between the fibre and the bark; 

 hence Dicotyledons are also described as Exogens 

 (Grr. exo, outside ; gen, root of gignesthai, to be produced) . 

 It is owing to this mode of growth that the shrubs and 

 trees of this class have their wood arrayed in " annual 

 rings," or concentric layers. Contrasted with the leaves 

 of the Monocotyledons, which are usually parallel- 

 veined and more or less linear, those of the Dicotyledons 

 are reticulate in venation (i.e., their veins form a net- 

 work), and also much varied in form. As a further 

 contrast between the two classes, it is observed that in 

 Dicotyledons the radicle put forth by the embryo 

 persists, and usually develops into a tap-root giving rise 

 to a branched root-system. This is not so in Mono- 

 cotyledons (see p. 193). The great diversity of floral 

 form leads on to much variety of fruit and seed, and the 

 inflorescence (the arrangement of the flowers on the 

 stem) is also greatly varied. 



At this juncture it will be advisable to consider some 

 forms of inflorescence. When the flowers spring from 

 the angle between steam and leaf the inflorescence is 

 said to be axillary; the Speedwells are an example. 

 When the flower stalk springs direct from the root and 

 bears no leaves, it is called a scape (Primrose). A spike 

 is a flower-stalk bearing a number of sessile (stalkless) 

 flowers; example, Verbena (Fig. 63). A raceme has 

 flowers arranged as in a spike, but each flower has a 

 simple stalk; example. Currant (Fig. 64). In a corymb 



