248 THE POWER OF RESISTANCE TO EXTREMES 



Pringsheim T , who traced the effects of exposure to concentrated sunlight 

 under the microscope. The action is restricted to the part illuminated, and 

 may result in changes or disturbances which are not necessarily immediately 

 fatal, and which may be capable of removal. Protoplasmic streaming is 

 stopped, and deformatory changes may occur in the protoplasm 2 . The 

 chloroplastids appear to be more sensitive than the rest of the protoplasm, 

 and may permanently or temporarily lose their power of photosynthesis, 

 in the former case usually being more or less completely bleached. The 

 pigment dissolved in the cell-sap of the staminal hairs of Tradescantia is 

 readily destroyed, as is also that in the yellow chromatophores of the ray 

 florets of Calendula officinalis. The pigments of other plants are, however, 

 more resistant, and concentrated sunlight exercises no direct effect upon 

 drops of tannin or fat, or upon starch-grains. 



The decomposition of the pigments and the destructive action of light 

 occur only, or at least in the first instance, in the presence of oxygen, and 

 are therefore delayed or arrested in indifferent gases 3 . This at once shows 

 that we are dealing with a photochemical action, and not with the result 

 of any heating action. The more refrangible rays are in fact more effective 

 than the less refrangible ones. Hence Pringsheim found that the action 

 of sunlight was only slightly weakened when passed through a solution of 

 cuprammonia, whereas the interposition of a solution of potassium bichromate 

 weakened the action considerably. The light became almost innocuous after 

 passing through a solution of iodine in carbon bisulphide, although the rays 

 which pass through this medium have the greatest heating power. Bacteria 

 react similarly, including those which are injured by diffuse daylight 4 . 



In general the most intense action appears to be exercised by those 

 rays which exercise most influence upon growth and movement, but it is not 

 certain whether the curves of action correspond exactly, and whether in 

 those cases where the less refrangible rays influence the process of growth 

 most, the same rays also exercise the most pronounced injurious effect 

 when concentrated. It is also uncertain whether the isolated green rays 

 exercise any special injurious action, either directly or by producing 

 functional disturbances leading to injury. 



The action is presumably not always the same, and the death of 



1 Pringsheim, Jahrb. f. wiss. Bot, 1879, Bd. xn, p. 288. Pringsheim used sunlight thrown by a 

 heliostat on a specially large microscope mirror and brought by a convex lens to a spot of light of 

 about 0-35 mm. diameter. Less than this is usually sufficient, and in sunlight increased 6 or 8 times in 

 photochemical intensity by an Abbe condenser, cells of Chara, Nitella> Elodea, &c., are bleached 

 in a few minutes. Cf. Ewart, Annals of Botany, 1898, Vol. xn, p. 384 ; Klemm, Jahrb. f. wiss. Bot., 

 1895, Bd. xxvm, p. 644; Pfeffer, ibid., 1900, Bd. xxxv, p. 711. 



8 Klemm, 1. c. 



8 Pringsheim, 1. c., pp. 351, 358. On bacteria cf. Fliigge, Die Mikroorganismen, 1896, Bd. I, 



P- 443- 



* Cf. Beck and Schultz, Zeitschr. f. Hygiene it. Infectionskrankheiten, 1897, Bd. xxm, p. 490. 

 [See also Phil. Trans., 1894, p. 961.] 



