228 PLANT PHYSIOLOGY 



This in itself is injurious, though the cells may recover their 

 turgor, becoming adjusted to the greater density. Plants 

 continual!}' exposed in nature to salt solutions of considera- 

 ble density present a very different appearance from those 

 growing under more usual conditions.* They are usually 

 fleshier and coarser, often more compact in habit, than ordi- 

 nary land plants ( compare Solidago sempervirens with the 

 other Solidagos of New England, the marsh grasses with 

 those of meadows, etc. ) . 



The theory of electrolytic dissociation of the atoms of 

 substances dissolved in proportionally large volumes of 

 water is extremely useful to the physicist and chemist. It 

 has been profitably employed by some physiologists in dis- 

 cussing the poisonous effects of certain substances ordinarily 

 harmless. The theory is useful, but it is very far from ex- 

 perimental proof. Nevertheless it may be employed in plant 

 physiology to suggest possible explanations of otherwise 

 incomprehensible phenomena. We may conceive the cell-sap 

 — an aqueous solution of a great number of substances in very 

 various and inconstant amounts — as containing not only 

 substances still in the molecular state, but also substances 

 in a state of atomic dissociation, in which the component 

 atoms are scattered at considerable distances from each 

 other throughout the solution. Whether these dissociated 

 atoms or groups of atoms smaller than molecules are 

 charged with electricity — some positive, others negative — or 

 whether the atoms possess only their chemical properties 

 and affinities is by no means known. Certain it is, how- 

 ever, if there are dissociated atoms in the cell-sap, that 

 these atoms are more susceptible to physical and chemical 

 influences within and without the ceU, and more prompt 

 and active in response, than are atoms associated in mole- 

 cules. Dissociated atoms exhibit the properties of the 

 chemical elements, whereas associated atoms present only 

 the properties of compounds. The elements are more ready 

 for combination than compounds are for recombination. A 

 dilute solution, one containing dissociated atoms, is there- 



* Schimper, A. F. W. Pflanzengeographle auf physiologischer Grund- 

 lage. Jena, 1898. 



