ELIMINATION 195 



(c) Excretion. 



The elimination of waste materials from the plant, whether 

 these are by-products of various phases of metabolism, or mate- 

 rials absorbed into the plant incidentally along with useful mate- 

 rials, has from the present point of view three phases, — the ex- 

 cretion of gases, of minerals, and of plastic or liquid substances. 



Excretion of Gases. 



This is a matter of considerable importance owing to the 

 fact that oxygen is a waste product in photosynthesis and carbon 

 dioxide in respiration. Yet the physics of the subject, in the 

 light of the foregoing studies, is very simple. The gases diffuse 

 from their places of release through the water of the protoplasm 

 to the surfaces of the cell walls, and thence diffuse from solu- 

 tion into the intercellular system, and thence outward through 

 the stomata, the energy being supplied by heat from the sur- 

 roundings. 



Excretion of Minerals. 



Of mineral matters useless to the plant there are consider- 

 able quantities, as to the nature of which the student should 

 inform himself. But plants have developed no system for get- 

 ting rid of them, and either store them up by a process phys- 

 ically the same as Secretion, in spare tissue, or else drop them 

 with bark, leaves, and other deciduous parts. The student 

 should inquire as to the known or supposed extent to which 

 plants actually transport substances into parts about to fall. 



Excretion of Plastic or Liquid Matters. 



This subject is almost wholly concerned with the possible 

 excretion of organic substances by roots, the possible formation 

 of acids by these belonging under Secretion, where it has already 

 been noted. The excretion of organic matters by roots, which 

 may ultimately prove a matter of high ecological importance, 

 has been most fully studied at the Bureau of Soils of the United 

 States Department of Agriculture, and may be found discussed 



