CHAPTER XXXIII 

 TROPISMS THE SLAVES OF THE LAMP 



" Behold the child, by Nature's kindly law, 

 Pleased with a rattle, tickled with a straw : 

 Some livelier plaything gives his youth delight, 

 A little louder but as empty quite ; 

 Scarfs, garters, gold, amuse his riper stage, 

 And beads and prayer-books are the toys of age. 

 Pleas VI with this bauble still, as that before, 

 Till tir'd he sleeps, and life's poor play is o'er." Pope. 



It may be laid down as axiomatic that once an object has come 

 into eqiiihbrium with its environment, it will remain so until some 

 change in the environment disturbs the harmony. In other 

 words, matter has inertia — moves when it is moved, stops when 

 it is stopped, and alters only in so far as it is altered. All in- 

 organic substances are not in equilibrium with their environment. 

 Radioactive minerals, as we have seen, are characterised by 

 tmdergoing considerable change — seemingly independent of the 

 nature of their surroundings. It is a moot point whether living 

 things may be brought under this general statement. That they 

 have inertia both in the ordinary physical sense and functionally 

 is undoubted. Moreover, that alterations in the distribution of 

 energy in the environment do lead to apparently corresponding 

 alterations in the organism will be granted by most workers in 

 this held, but all are not agreed as to how far the interpretation 

 can be applied to animals high in the scale. 



I. One of the best investigated phenomena in this line of study 

 is that of heliotropism or phototaxis. It is well known that 

 radiant energy is capable of influencing the rate of some chemical 

 reactions — in proportion to the intensity of the light. This is 

 known as the Bunsen-Roscoe Law, which may be formulated as : 



it — constant, 



where i is the intensity of the light and t the time of exposure. 



(1) Certain animals and free-swimming plant-organisms move 

 towards or away from the source of light. These are said to be 

 positively or negatively heliotropic respectively. Caterpillars of 

 Porthi'sia chrysorrhoea are of the former class. They move towards 

 the light and may starve, wdth abundance of food just behind them. 



(2) If positively and negatively heliotropic animals are placed 



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