ACTION OF LIGHT ON VEGETATION. 
757 
endowed with motion'^. The leaf-blades of Leguminosse, Oxalideae, Marantaceae, 
Marsiliaceae, &c. are borne on modified petioles which serve as contractile organs, 
bending upwards or downwards under various external and internal influences, 
and thus giving a variety of positions to the leaf-blades. If these plants are 
placed in permanent darkness, the curvatures due to internal changes alternate 
upwards and downwards. Light exercises an immediate influence on these peri- 
odically contractile organs ; any increase of its intensity tends to give the blade an 
expanded position, such as it occupies in the day-time \ any diminution tends to 
cause it to assume a closed position upwards or downwards such as it has in the 
night. This effect, which I formerly termed '• the paratonic action of Hght,' is not 
the cause of the periodic movements; but rather counteracts the periodicity caused 
by the internal forces. In most leaves endowed with periodic movements the para- 
tonic influence of light is so strong that it neutralises them, and induces in their 
place a periodicity dependent on the alternation of day and night. In the lateral 
leaflets of the leaves of Desmodium gyrans on the contrary the internal causes of the 
rapid periodic oscillations are so powerful as to overcome the paratonic action of 
light; and these leaflets move upwards and downwards when the temperature is 
high even in spite of changes in the amount of light. My earlier researches ^ show 
that it is only the more refrangible rays that produce a paratonic effect, while red 
rays act Hke darkness. 
The influence of light on the position of the contractile organs is not however 
only of this direct character; the motile condition is also indirectly dependent on 
it. Both the periodic and paratonic movement, as well as that {Mimosa) due to 
mechanical irritation — in fact, the power of movement — is lost when the leaves 
have remained in the dark for a considerable time, such as a whole day ; in other 
words, they become rigid by long exposure to darkness. From this rigid condition 
they do not immediately recover when again exposed to light ; the exposure to light 
must continue for a considerable time, some hours or even days, before the motile 
condition which I have termed * Phototonus ' is restored. It is only in this condition 
that the leaves are motile and sensitive to changes in the intensity of the light 
or to mechanical irritation. The paratonic curvatures of fully developed contractile 
organs caused by the action of light are distinguished from the heliotropic curvings 
of growing organs by the fact that, firstly, they are connected with phototonus, 
while the latter are not ; and secondly, that they always take place in a plane 
determined by the bilateral structure, while the plane of heliotropic curvature depends 
only on the direction of the rays of light. 
\The Chemistry of Chlorophyll. Gautier (Comptes Rendus, 1879, and Bot. Zeitg. 1880) 
and Hoppe-Seyler (Ber. deut. chem. Ges. 1879, and Bot. Zeitg. 1879) have succeeded in 
obtaining green crystals on the evaporation of an alcoholic solution of chlorophyll. 
Gautier considers these crystals to consist of chlorophyll, but Hoppe-Seyler is of opinion 
that they are a modification of chlorophyll, to which he gives the name of chlorophyllan. 
The following are the analyses : — 
^ See Sachs, Ueber vorübergehende Starrezuslände, &c., Flora, 1863. — Further details will be 
given HI Chap. IV. [Also Darv^'in, Movements of Plants, i88d.] 
^ Sachs, Ueber die Bewegungsorgane von Phaseolus und Oxalic, Bot. Zeit. 1857, p. 81 1 e: if'/. 
