170 Plant Life and Evolution 



to develop permanent woody stems is a marked char- 

 acter of the class, and this is due to the development 

 of cambium, or permanent growing tissue, in the 

 woody bundles of the stem, very much as in the 

 conifers, and in the stems of some ancient pterido- 

 phytes. This gives them the advantage of a per- 

 manent growth of the stems, which do not have to 

 be formed anew each year, as is so commonly the 

 case with monocotyledons. 



With the greater diversity of the stem structure 

 goes an equally diversified leaf type. The leaves 

 may be either simple or divided, and in many cases 

 elaborately dissected in a variety of ways. The 

 great adaptability of the dicotyledonous type is well 

 shown by the history of various weeds. While some 

 of the weeds are monocotyledons, the grasses rank- 

 ing first in this category, many more weeds are 

 dicotyledons, which often have been brought from 

 remote lands but have very quickly made them- 

 selves at home, not infrequently driving out the 

 native flora. Climbing plants, carnivorous plants, 

 epiphytes, parasites, and aquatics are some of the 

 types that have been evolved among the dicoty- 

 ledons. 



Flowers of Dicotyledons. The flowers of the 

 dicotyledons (Fig. 18) are far too various in their 

 structure to permit of anything more than the brief- 

 est sketch of their more salient features. Like the 

 monocotyledons there are two main categories, apet- 

 alous and petaloideous forms. A number of very 



