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 



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- 



FiG. 48. — A, a seedling of the 

 castor-bean (Riciiius) , showing 

 tlie difference in appearance 

 between tlie two cotyledons, 

 cot, and the second leaves, I; 

 r, tlie 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 tlie whole 

 seed-cavity ; co(, cotyledons ; 

 St, stem ; D, section of the seed 

 of blood-root (Sanguinaria), 

 showing the small embryo, em. 



