446 MODIFIED CIECUMNUTATION. Chap. Vnt 



a bright sun, they stood at 56° above the horizon ; they were 

 then protected from the rays of the sun, but were l^ft weU 

 illuminated from above, and after 30 m. they had fallen 40°, for 

 they now stood at only 16° above the horizon. Some young 

 plants of Fhaseolus Hernavdesii had been exposed to the same 

 bright sunlight, and their broad, Tinifoliate, first leaves now 

 stood up almost or quite vertically, as did many of the leaflets 

 on the trifoliate secondary leaves ; but some of the leaflets had 

 twisted round on their own axes by as much as 90° without 

 rising, so as to present their edges to the sun. The leaflets on 

 the same leaf sometimes behaved in these two different manners, 

 but always with the result of being less intensely illuminated. 

 These plants were then protected from the sun, and were looked 

 at after W h. ; and now all the leaves and leaflets had re- 

 assimied their ordinary sub-horizontal positions. The copper- 

 coloured cotyledons of some seedlings of Cassia mimosoides were 

 horizontal in the morning, but after the sun had shone on 

 them, each had risen 45^° above the horizon. The movement 

 in these several cases must not be confounded with the sudden 

 closing of the leaflets of Mimosa pudica, which may sometimes 

 be noticed when a plant which has been kept in an obscure 

 place is suddenly exposed to the sun ; for in this case the light 

 seems to act, as if it were a touch. 



From Prof. Wiesner's interesting observations, it is probable 

 that the above movements have been acquired for a special 

 purpose. The chlorophyll in leaves is often injured by too 

 intense a bght, and Prof. Wiesner * believes that it is protected 

 ■by the most diversified means, such as the presence of hairs, 

 colouring matter, &c., and amongst other means by the leaves 

 presenting their edges to the sun, so that the blades then 

 receive much less light. He experimented on the young leaflets 

 of Eobinia, by fixing them in such a position that they could 

 not escape being intensely illuminated, whilst others were 

 allowed to place themselves obliquely ; and the former began to 

 suffer from the light in the course of two days. 



In the cases above given, the leaflets move either upwards 



* 'Die Naturlichec Einrich- the action of concentrated light 



tungen zura Schutze des Cliloro- from the sun, in the presence of 



piiylls,' &c., 1876. Prinssheim oxygen. Sep, also, Stahl on the 



has recently observed under tlie protection of chlornphvll from 



microscope the destruction of intense light, in 'Bot. Zeitun", 



chlorophyll in a few minutes by 1880. °' 



