MOVEMENTS OF PLANTS IN THE LIGHT. 99 



of animals, no matter whether the mechanism be the lungs of a mammalian or the 

 branchial organs of fishes. Every variety of breathing apparatus has for its object the 

 evolution of heat by the oxidation of carbon, or of carbon and hydrogen conjointly ; 

 but the object of the agency of leaves upon the air is to obtain from it carbon, or car- 

 bon and hydrogen. At certain periods of their history, plants themselves become ma- 

 chines of combustion, when the process of fertilization requires that for a time, in a 

 given place, and for a specific object, there should be an elevation of temperature. Re- 

 sort is then had to those same processes which obtain in animal systems ; and sugar, 

 or what comes to the same thing, honey, is burned. 



CHAPTER XII. 



ON THE MOVEMENTS OP PLANTS UNDER THE INFLUENCE OF THE DIFFERENT RAYS OF 



LIGHT. 



CONTENTS: Movements described. Produced by Artificial Light. Experiment of Dr. 

 Poggioli. Experiments of Dr. Gardner, M. Payer, and a Committee of the French 

 Academy. Movement takes place in two Directions, \st, towards the Light ; 2d, Lat- 

 erally. Thejirst Motion is produced by the Indigo Ray. Tables of Direct and Lat- 

 eral Movement. Lateral Motion commences in the Yelloiv. 'Connexion beticeen the 

 Colour of the Sky and the Vertical Position of Stems. 



The Roots recede from the Violet Light. Movement begins in the Violet. Lateral 

 Flexure of the Roots. It is towards the Red. It commences with the Yellow Ray. 



388. THE tendency exhibited by the parts of plants to move under the influence of 

 light has been known for a long time. Such motions are produced in a striking man- 

 ner when a beam enters a dark cellar in which plants are caused to grow, their stems 

 directing themselves to the luminous place. Under other circumstances a reverse mo- 

 tion ensues, and the vegetable parts, instead of moving to the light, seem to retreat 

 from it. 



389. Artificial light, such as that of an Argand lamp, can determine similar move- 

 ments in stems that are very flexible. A crop of young turnips, which has been caused 

 to grow in the dark, when set before a lamp soon exhibits the phenomenon in ques- 

 tion. Each plant is disturbed from its perpendicular position, and is caused to incline 

 to the rays. While this result seems to hold very generally with the stems, it is differ- 

 ent with the roots. When plants are caused to be developed on the surface of water 

 contained in glass vessels, in many cases their roots retire from the light, as M. PAYER 

 has observed in the cabbage and white mustard. This action appears to take place 

 upon the spongiole, which is the only part increasing in length, so that as it grows the 

 rootlet curves. The curvature differs entirely from that of the stems, in the case of 

 which it is only transient. Stems which have been disturbed from their perpendicu- 



