474 



Minnesota Plant Life. 



leaves are the principal succulent organs; and stem-succulents, 

 such as the cacti, in which the leaves are reduced to thorns and 

 the stems have become fleshy. Of adaptational types there 

 are the following: desert-succulents, generally armed with 

 thorns; rock-succulents, more commonly unarmed; salt-suc- 

 culents, like the glassworts, sometimes armed; and tree-top- 

 succulents, generally unarmed. Of the latter group tropical 

 orchids furnish illustrations. Some of the leaf-succulents de- 

 velop their leaves in rosettes and belong also to the class of 



FIG. 233. Cottonwood trees on the Minnesota river. After photograph by Williams. 



rosette vegetation; the century-plants, the live-forevers, and 

 the little hen-and-chickens are examples of this group. The 

 tree-top-succulents, also, of the orchid family, to be seen in 

 greenhouses, in a number of instances develop their leaves in 

 clusters upon short stems. Other leaf-succulents, however, 

 have the stems slender and branching. This is true of the 

 purslane, the rock purslane and the Russian thistle. Succulents 

 may have arisen through direct influence of the environment, 

 by the warming of the soil. This would tend to increase the 



