BOTANY 



Nyctitropic Movements. The sleep-movements, or nyctitropic 

 movements, exhibited by many leaves and flowers, are usually the 

 result of changes of turgor in the motile organs. Such leaves, which 

 are especially common among the Leguminosae (Trifolium, Phaseolus, 

 Robinia, Mimosa, etc.), usually show a well-developed pulvinus. 

 The significance of these sleep-movements, in which the leaves are 

 closely folded together, is a diminution of the leaf surface exposed 

 to radiation, thus protecting the plant against loss of heat. 



The effect of light upon growing organs is generally to check the 

 growth in length, so that, other things being equal, plants grow more 



actively at night than by day. 

 The accelerating effect of dark- 

 ness upon growth in length 

 is seen in the extraordinary 

 elongation of etiolated shoots 

 of potatoes sprouted in a 

 cellar, or the spindling growth 

 of house plants grown in 

 insufficient light. If light is 

 too intense, it may completely 

 arrest growth for the time 

 being. Experiments have 

 shown that the more highly 

 refracting rays of the blue 

 end of the spectrum are much 

 more effective in retarding 

 growth than are the red and 

 yellow rays. 



Heliotropism. In general, 

 green shoots are positively 

 heliotropic, but occasionally 

 (Ivy) they are negatively 

 heliotropic, this being, pre- 

 sumably, an -adaptation connected with the attachment of the 

 tendrils. Among the lower plants, green organs are as a rule 

 positively heliotropic, and within the cell, the movements of the 

 chromatophores are regulated by the direction and intensity of the 

 light. The sporangiophores of Moulds, and the stalked spore-bodies 

 of such large Fungi as Toadstools, are often strongly heliotropic. In 

 the Ferns, the position of the archegonia is dependent upon illumi- 

 nation, and they may be made to develop upon the upper side of the 

 prothallium, if the latter is illuminated from below. 



The position of the leaf is most commonly horizontal, but in case 

 the light is excessive the leaf may shift its position to avoid the 

 direct rays of light. When a leaf is motile, as in Robinia and other 



FIG. 463. Twig of Manzanita (Arctostaph- 

 ylos Manzanita), showing vertical posi- 

 tion of the leaves. 



