I 



PHYSIOLOGICAL 327 



an animal nerve. Moreover, the idea of a reflex arc is boldly extended 

 from the animal to the plant. In the animal a stimulus excites the 

 ending of a sensory nerve-fibre, the thrill passes to the sensory nerve- 

 cell, thence to an associative or communicating nerve-cell, thence 

 to a motor nerve-cell, and thence by a motor nerve-fibre to the 

 muscles which contract. So, before we can say "reflex action", we 

 draw our finger away from a hot object. But there is a similar 

 phenomenon, according to Sir Jagadis, in the Sensitive Plant. If a 

 very slight stimulus be applied to one of the secondary ribs of a leaf, 

 it gives rise to an ingoing impulse causing the characteristic response 

 in a part of the motor cushion or pulvinus; and that is all. But if 

 there be a slight increase in the intensity of the stimulus, a new 

 phenomenon makes its appearance ; the ingoing or afferent impulse, 

 reaching the centre (perhaps a question-begging term), becomes 

 reflected along a new path as an efferent impulse. If these conclusions 

 are established up to the hilt, the work of Sir Jagadis is of the 

 highest importance, for it means not merely that phenomena analo- 

 gous to animal reflex actions occur in plants, as has been often 

 suggested, it means that plants may have nervous reflex arcs pre- 

 cisely comparable to those of animals. 



Whatever conclusion be reached on this crucial question we 

 cannot but admire the ingenuity of the experimenter's methods. 

 Thus by a refinement of the electrical stimulation he secures a 

 controlled amount of stimulus. Thus, again, he has improved the 

 methods of recording both the leaf movement and the electrical 

 responses, so reaching a more exact timing of the rates of conduction 

 and reaction. Science, it has been said, begins with measurement; 

 the advance Sir Jagadis has made shows how it is continued by 

 measurement. 



Among the new facts brought forward in this continuation of his 

 always suggestive and brilliant researches, three may be noted: 

 (i) The motor cushion at the base of the leaf-stalk has four different 

 quadrants which bring about different movements — up and down, 

 left and right. More than that, there is a definite nerve-connection 

 between each of the four leaflets and a corresponding quadrant of 

 the motor cushion. This is very striking, not to say strange. 



(2) The movement of the leaf of the Sensitive Plant is brought 

 about by changes in turgor (or intracellular pressure) in the motor 

 cushion or pulvinus. When a stimulus is applied directly to the 

 cushion, the leaf falls in about o • i of a second. If the stimulus be 

 indirect, i.e. at some distance from the pulvinus, there is a longer 

 interval before collapse, and during this interval there is a weak 

 upward movement, which Sir Jagadis calls erectile. Generalising 

 from this, too quickly perhaps, he defines the effect of direct 

 stimulation as a diminution of turgor, consequent contraction, 

 diminished rate of growth, and the negative response — the sinking 



