ACTION OF LIGHT ON VEGETATION. 751 



and chlorophyll-granules collect chiefly on the walls that are not free, i. e. on those 

 adjacent to other cells. The occurrence of apostrophe under direct sunlight which 

 Borodin asserts* (in various Phanerogams, as Lemna, Callitriche^ and Stellaria), is 

 denied by Frank, who maintains that what takes place in these cases is rather a 

 collection of the protoplasm at the spots where the light is strongest, which may 

 happen to be at the sides^. 



It is evidently these aggregations of chlorophyll-granules on the side-walls of 

 the cells caused by sunlight which were observed by Borodin that produced the 

 phenomenon pointed out by Marquard and more exactly described by myself, 

 VIZ. that green leaves (e.g. those o^ Zea, Pelargonium^ Oxalis, Nicoiiana, Sec.) when 

 exposed to sunlight assumed a bright green colour in a shorter time than in 

 diffused light or in deep shadow. This can be made very evident by shading 

 particular parts by pressing closely on them a strip of lead or tinfoil ; if this 

 strip is removed after five or ten minutes, the parts that were shaded show a 

 dull green, those exposed to the sun a bright green colour. It is obvious that the 

 tissue will appear to the eye a deeper green in proportion as the green granules are 

 distributed uniformly over the surfaces facing the eye, a less deep green in propor- 

 tion as they collect on the side-walls. Borodin's observations directly confirm this 

 hypothesis. This alteration in the grouping of the chlorophyll-granules which 

 accompanies a change in the intensity of the light is caused only by the highly 

 refrangible rays; the less refrangible rays (the bright and red ones) have the same 

 eff"ect as darkness ^ It results therefore, as I showed in 1859, that if a strip of 

 blue glass is laid on a leaf exposed to sunshine, it will produce no change of colour, 

 while one will be caused by a strip of red glass. 



Since these movements of the chlorophyll-granules are produced by the 

 colourless protoplasm in which they are imbedded, it might be expected that the 

 protoplasm of hairs which contain no chlorophyll or only a small quantity would 

 be similarly influenced by the colour and intensity of the light. But the state- 

 ments of Borscow and Luerssen^ which might be interpreted in this direction at 

 least to some extent have not been confirmed by the observations of Reinke^. 



The swarming of zoogonidia is also connected with protoplasmic movements. 

 Their motile organs, the cilia, are supposed to be slender threads of protoplasm, by 

 the vibration of which both the rotatory and the advancing movement of the zoogo- 

 nidia is caused. The axis of rotation becomes subsequently the axis of growth ; the 

 anterior end in the advancing motion (where the zoogonidium is usually narrower, 



■■ Borodin, Melanges biol., Petersburg 1869, vol. VII. p. 50. 



^ [From Stahl's investigations it appears that apostrophe is produced by direct sunlight (Bot. 

 Zeitg, 1880). He finds that exposure to diffuse daylight produces epistrophe, that is, the position of 

 the chlorophyll-granules in which the greatest area of their surface is exposed to the incident rays, 

 M'hereas sunlight produces apostrophe, that is, the position in which the least possible area of their 

 surface is exposed to the incident rays. In the one case they present their flat surfaces, in the other 

 their edges to the incident rays.] 



3 Sachs, Berichte der math.-physik. Klasse der k. sachs. Ges. der Wiss. 1859. 



* Borodin, l.c, Frank, Bot. Zeitg. 1871, p. 238. 



5 Borscow, Melanges biol., Petersburg 1867, vol. VI. p. 312. — Luerssen, Ueber den Einfluss des 

 rothen u. blauen Lichts u. s. w., Di!^sertation, Bremen, 1868. 



^ Reinke, Bot. Zeitg. 1871, Nos. 46, 47. • 



