STIMULI AND THEIR ACTIONS 399 



a. The Phenomena of Excitation 



The whole organic world of to-day is directly dependent upon 

 the metabolic action of light. The old philosophers of nature, as has 

 been seen, characterised animals, in a certain sense not incorrectly, 

 as parasites upon plants. It is true that carnivora nourish 

 themselves upon animal substances ; but this animal food is derived 

 from herbivora, and thus the carnivora also are thrown back upon 

 the plants. But plants cannot exist without the influence of 

 light. The sun's rays give the stimulus that causes the chlorophyll 

 bodies of the plant-cells to decompose the carbonic acid of the air 

 into carbon and oxygen, and from the carbon, with the water taken 

 in through the roots, to produce synthetically the first organic 

 substance, the first product of assimilation, starch. Further, the 

 sun's rays also give the impulse to the production of the green 

 chlorophyll colouring-matter itself; this follows from the fact that 

 plant seeds, sprouting in the dark, produce a white or bright-yellow 

 plant, which grows for a time at the expense of the reserve- 

 substances stored up in the plant seed, but which becomes green 

 only when exposed to the light. Only after it becomes green is 

 the plant able to decompose carbonic acid and form starch. Thus, 

 the first organic product, from which all other organic substance is 

 derived, originates from the action of the photic stimulus of the 

 sun's rays. 



This assimilatory action of sunlight does not belong to all light- 

 rays in equal measure. As has already been seen, 1 with equal 

 intensities the red rays have the strongest action. 



As regards most of the objectively perceptible effects of light 

 upon the retinal cells in the eyes of man and of animals, so far it 

 is uncertain whether they depend upon the direct stimulation 

 of the cells in question, or upon reflex stimulation through the 

 central nervous system. Nevertheless, metabolic effects must be 

 present in the retinal cells, since their results in the central nervous 

 system, to which the excitation is transmitted through the optic 

 nerves, we subjectively feel as colours, and objectively recognise in 

 other men or animals in the movements that are called out by 

 photic stimulation through the mediation of the central nervous 

 system. 



As regards the excitation-effects of light upon form-changes, 

 thus far nothing is known. 



Numerous effects upon changes of energy, especially in motile 

 phenomena, have been recognised. 



In certain fresh-water ponds there is found concealed between 

 mud and sand, in almost total darkness, an awkward, sluggish, 

 amoeba-like rhizopod, Pelomyxa. The lumpy, naked protoplasmic 



1 Cf. P , 217. 



