190 8- 1909.] The Sense- Orgaris of Plants. 1 69 



knowledge of the subject. In dicotyledonous seedlings the 

 cotyledon and upper part of the hypocotyl have been shown 

 to be the perceptive region. Eoots are either non-perceptive 

 or only possess photo-irritability to a slight extent. The 

 latter is recorded for roots of Sinapis, Lepidium, and Heli- 

 anthus, which are weakly negatively heliotropic. 



To demonstrate the photo - irritability of dicotyledonous 

 seedlings the following experiment serves. Mustard-seeds 

 are planted in a moist chamber with a lateral aperture 

 which is at first closed. An inverted flower - pot serves 

 this purpose. After the seeds have germinated the window 

 is opened, with the result that the apices of all the seed- 

 ling stems curve towards it, and finally grow out (Plate XII., 

 fig. 2). 



Plate XIII., figs. 1, 2, illustrate the experiment in proof of 

 the localisation of photo-perception. Of three seedlings which 

 have been subjected to unilateral illumination until a decided 

 curvature is produced, one, A, has its cotyledons and upper 

 part of the hypocotyl covered with tinfoil ; in B, the basal 

 part of the hypocotyl is swathed in tinfoil; c is a control. 

 All are again unilaterally stimulated on the convex side ; A 

 alone shows no curvature, — its perception area being covered, 

 it has been unable to perceive the stimulus. 



Ordinary growing leaves orient their upper surface at right 

 angles to the incident rays of light, and thus assume a fixed 

 light-position. Cases occur where adult leaves possess the 

 power of altering their light-position in relation to variations 

 in the light incidence : they in addition may execute move- 

 ments in response to variations in the light intensity. Mimosa 

 and other Leguminosse supply familiar examples of leaves 

 possessing this property, which is associated with the occur- 

 rence of special motile pulvini in the leaves. These pulvini 

 are both perceptive and motor organs. Ewart ('Ann. of 

 Bot.,' 1897, xi. 447) has shown that the main and secondary 

 pulvini of Mimosa leaves perceive and respond to variation in 

 the light direction, so that the leaf as a whole is diaheliotropi- 

 cally oriented : the pulvini of the leaflets perceive and respond 

 to variations in light intensity, and so determine the opening 

 and closing of the leaflets, irrespective of the position which 

 the leaf as a whole may occupy. 



