Macfarlane. The Sensitive Movement« of some Flowering Plants. 181 



Leg um in os ae, and that broadly speaking the sensitiveness increases 

 as we pass froms regions where the sun's rays are of minor inten- 

 sity, to others where the rays are of increased intensity. 



The writer therefore regards the action of the more refran- 

 gible rays when of a definite intensity as one of Stimulus, because 

 1) the angular inflexion of leaflets is proportionate to the inten- 

 sity of the stimulating rays ; 2) the movement is not due to 

 indirect action from the green laminar substanoe to the pulvinus 

 cells, but is wholly centred in the latter; 3) if the inflexion move- 

 ment is considerable, the white cushion of the pulvinus shows a 

 visible change from white to a dull leaden green color; 4) when 

 the more refrangible rays are cut off by a color screen the Stimulus 

 is removed, and then neither the heat rays nor the less refrangible 

 .light rays cause closure. 



Under the lirst head it is worthy of note here that the 

 parallelism between mechanical, thermal and chemical Stimuli and 

 Stimulus by the more refrangible rays is complete , a weak 

 Stimulus causing slight inflexion and a strong Stimulus powerful 

 inflexion. 



Under the third head it should be stated that the same mole- 

 kular change takes place in the pulvinus region of Cassia nictitans 

 and C. chamaecrista as I have already demonstrated to occur in 

 species of Mimosa and Schrankia*). After mechanical Stimulus 

 a latent period of fully five seconds in C. nictitans ensues before 

 this change occurs, but then the difference between a stimulated 

 and non- stimulated leaflet is striking. By careful Observation it 

 can be shown that the same change occurs from light Stimulus by 

 the more refrangible rays. 



The above experiments then indicate that by the parahelio- 

 tropic movement leaflets are protected from the intense action of 

 the blue-violet rays, and for this end all of the leaflets on any 

 one leaf need not move through the same angle. Oltmanns 

 (p. 238) says : „Die Blätter der Robinia sind in hervorragendem 

 Maasse photometrisch; jede Veränderung der Lichtintensität be- 

 antworten sie mit einer veränderten Stellung der Spreite" .... 

 „das Blatt lässt Licht von bestimmter Intensität unter einem genau 

 definirten Winkel auf sich wirken. Der Lichteinfallswinkel hängt 

 wieder von der Stimmung der Blätter ab." 



Those cases are not rare where even adjoining leaflets show 

 very diverse movements that it would be difficult to explain on 

 the above grounds. Thus on a bright, warm day a plant of 

 Oxalis striata was observed at 2:15 p. m. having several leaves 

 whose leaflets were in pronounced paraheliotropic position, but 

 one leaf had its petiole so placed that a leaflet next to the in- 

 cident rays had bent down through an angle of 78°, another 

 away from the source of light by falling only 25° then had its 



*) Biological Lectures at Wo od 's Holl. 1K93. p. "205. 



