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. 23iJ. Cottonwood trees on llu- Minnesota river. .Vfter photograpli by Williams. 



rosette vegetation ; the century-plants, the li\'e-forcvers, 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 Ivussian thistle, v^ucculents 

 may have arisen through direct influence of the onviroinnent, 

 bv the warniinu" of the soil. This would tcml to increase the 



