200 EVOLUTION OF PLANTS 
groups of Angiosperms, although at present no such 
forms are certainly known. 
The Dicotyledons exhibit great variety in the form of 
-- cot 
Fie. 48.— A, a seedling of the 
castor-bean (Ricinus), showing 
the difference in appearance 
between the two cotyledons, 
cot, and the second leaves, 1; 
7, the main or tap-root, a contin- 
uation of the stem; B, cross- 
section of the stem, showing 
the arrangement of the tissues ; 
vb, the vascular bundles; C, 
section of the seed of the 
shepherd’s purse (Capsella) , the 
embryo occupying the whole 
seed-cavity; cot, cotyledons; 
st, stem; D, section of the seed 
of blood-root (Sanguinaria), 
showing the small embryo, em. 
the stem and leaves, and this 
is correlated with a much 
more perfect development of 
the tissues than is found 
elsewhere in the vegetable 
kingdom. This is shown 
especially in the highly de- 
veloped vascular bundles, 
which in the stems of the 
woody forms show a second- 
ary thickening like that in 
the coniferous stem, but the 
tissues of the bundle are 
much more specialized than 
in the latter. From the 
continued growth of the 
cambium or active tissue in 
the bundles of the stem, an- 
nual growth-rings result, and 
soon the greater part of the 
stem is made up of the sec- 
ondary wood derived from 
the activity of the cambium. 
This results in the develop- 
ment of the massive woody 
stems characteristic of dico- 
tyledonous shrubs and trees. 
In this secondary thickening of the stems and roots, the 
Dicotyledons differ from the Monocotyledons and ap- 
