The Plant. 83 



are notably efficient as phosphorus gatherers, while the grass 

 family is characterized by high assimilation of silicon. 



The well known power of roots to etch the surface of lime- 

 stone is due to excretion of carbon dioxide from this organ of the 

 plant, and differences in ability to assimilate food materials may 

 be explained partly by differences in carbon dioxide output. 



If the stem of an actively growing plant be severed at its junc- 

 tion with the root and replaced by a pressure gauge, it will be 

 found that the root exerts an upward pressure amounting in some 

 cases to more than 30 pounds per square inch. According to 

 Wieler this pressure has been found sufficient to support a column 

 of water of the following heights in the plants indicated : 



Height of water column 

 Plant supported by root pressure 



White Mulberry 6.5 inches 



k European Ash 11.4 " 

 Castor Oil Plant 181 .3 " 

 Stinging Nettle 249.7 

 Wine Grape 581.6 " 

 White Birch 755.0 

 Sweet Birch (Black Birch) 1043.2 " 



By this so-called "root pressure" the root is believed to func- 

 tion in the movement of water through the plant. 



In biennial root crops such as the beet, the root of the first 

 year's growth serves as a magazine for food from which the 

 second year's growth is re-inforced for the production of seed. 

 This reinforcing material is usually starch or sugar, with small 

 amounts of nitrogen compounds and ash constituents. 



The stem. The active portion of the stems of plants consists 

 essentially of a system of tubes formed by continuously connected 

 cells. These tubes serve as channels for the transportation of 

 water and food materials and are surrounded by protecting and 

 supporting tissue. In the stems of endogenous plants, as in the 

 corn and the bamboo, the tough, smooth bark is formed by ag- 

 gregates of the dead remains of conducting cells and newer 

 growths are added by increments of these cejls in the soft pith 



