24 LIVERPOOL BIOLOGICAL SOCIETY. 
derived from external heat, and therefore that plants cool 
the bodies in their vicinity; but no experimental proof is 
given, nor is there any reference there or elsewhere to any 
such proof. So I think we must in the meantime confine 
the action of external heat to the maintenance of tem- 
perature, and refer the source of the whole kinetic energy 
required to light and those oxidative processes which also 
take place in plants. External heat is therefore only a 
condition of life, and does not supply the kinetic energy 
required for its functions. Water is also a condition, 
for it 1s not (except in chlorophyll bearing organisms) 
decomposed and used as pabulum. It is essential to 
maintain the requisite mobility of the protoplasm, which, 
though not soluble or diffusible, is capable of imbibition 
of such a large quantity of water that it has the 
mobility of a liquid, or rather, as Berthold maintains, the 
physical properties of an emulsion. The maintenance 
of the requisite osmotic state is lkewise a necessary 
condition of the environment. In many vegetable cells 
the protoplasmic part of the contents, which is always 
alkaline, is surrounded with faintly acid non-living cell- 
sap. During life these are kept in equilibrium, but any 
cause of diffusion of the alkaline contents produces decom- 
position and death of the protoplasm. Even distilled 
water may act thus prejudicially on the bioplasts of 
animals, and accordingly in experiments on animals the 
circulation cannot be kept up with pure water, but a weak 
solution of common salt is requisite. 
The third division of the environment consists of the 
stimuli, which act as liberators of vital action, and thus 
their effect appears out of proportion to their quantity. 
The stimul consist of the active forces and of chemical 
substances. ‘The forces thus acting are mechanical force, 
sound, light, electricity, vis nervosa and heat. The‘ 

