5J8. MODIFIED CIBCUMNUTATION. Chap. Vlil. 



CHAPTEE Vin. 



Modified Cibcumnctation : Movements exoited bt Light. 



Uiiitinction between heliotropism and the effects of light on the perio- 

 dicity of the movements of Itaves — Heliotropic movements of Beta, 

 Solanum, Zea, and Avena — Heliotropic movements towards an 

 obscure light in Apios, Brassioa, Phalaris, Tropseolum, and Cassia 

 - — Apbeliotropio movements of tendrils of Bignonia — Of flower- 

 peduncles of Cyclamen — Burying of the pods — Heliotropism 

 and apheliotropism modified forms of circumnutation — Steps by 

 wliich one movement is converted into the other — Transversal- 

 heliotropismus or diaheliotropism, influenced by epinasty, the 

 weight of the part and apogeotropism— Apogeotropism overcome 

 during the middle of the day by diaheliotropism— Eftects of the 

 weight of the blades of cotyledons — So-called diurnal sleep — Ciiloro- 

 phyll injured by intense light — Movements to avoid intense light. 



Sachs first clearly pointed out the important dif- 

 ference between the action of light in modifying the 

 periodic movements of leaves, and in causing them to 

 bend towards its source.* The latter, or heliotropic 

 movements are determined by the direction of the light, 

 whilst periodic movements are affected by changes in 

 its intensity and not by its direction. The periodicity 

 of the circumnutating movement often continues for 

 some time in darkness, as we have seen in the last 

 chapter ; whilst heliotropic bending ceases very quickly 

 when the light fails. Nevertheless, plants which have 

 ceased through long-continued darkness to move pe- 

 riodically, if re-exposed to the light are still, according 

 to Sachs, heliotropic. 



Apheliotropism, or, as usually designated, negative 



• • Physiologie Veg ' (French Translation), 1S68, pp. 42, 517, &o. 



