ABSORPTION 163 



The student should here also ascertain the present state of 

 knowledge or opinion as to the relative values of physical and 

 of chemical properties of the soil in determining plant habit 

 and distribution. He will find a strong argument against the 

 prevailing views given by FERNALD in Rhodora, 9, 1907, 149. 



Another phase of absorption is that by parasites, saprophytes, 

 and insectivorous plants in taking the organic matters from their 

 respective sources, and upon the physics of this absorption the 

 student should also inform himself. 



Such are the modes of absorption of water and minerals. It 

 remains to consider the absorption of gases, a matter of vast 

 importance because of the indispensable part played by carbon 

 dioxide and oxygen in the economy of the plant. Nitrogen, of 

 course, is not here in question, since it is absorbed only in com* 

 bined form in solution. A certain restricted amount of all three 

 gases is absorbed in solution in water through the roots, but this 

 is insignificant in amount, and, moreover, is stopped in the ves- 

 sels, thus explaining the gases always present there. For its 

 principal supply of carbon dioxide and oxygen, therefore, the 

 plant must draw upon the great reservoir, the atmosphere, which 

 it must absorb through its aerial parts. For a full understanding 

 of this subject the student must now renew his acquaintance 

 with the structure of the gas-absorbing parts, including the aera- 

 tion system, of the higher plants, together with the stomata and 

 lenticels, their distribution and connection with the intercellular 

 system, the morphology of the latter, its extent, its continuity 

 throughout the plant, its presence in compact tissues, and the 

 character and condition of the cells in contact with it. All these 

 matters the student should understand with a definiteness per- 

 mitting the construction of a generalized or conventional dia- 

 gram of the aeration system of the plant from root tip to stem 

 tip and leaf. So important to our present subject is the con- 

 tinuity of the aeration system and its connection with the stomata, 

 that it deserves some experimental study, which may take the 

 form of demonstration as follows : 



SUGGESTED EXPERIMENTS. Detach from its plant a long-petioled loose- 



