August 10, 1882] 



NATURE 



;55 



bladder?, each of which is distended with liquid. And its ten- 

 dency to expand as a whole is due to the tendency to expand of 

 the innumerable cells of which it is made up. In the unmu- 

 tilated state these are squeezed into a smaller spice than that 

 which they would assume if they were left to themselves ; and, 

 consequently, as their expansion is prevented, or curbed on one 

 »ide, it acts on the opposite side, so as to bend the cylinder in 

 the direction of the restraint. 



All of this we can, perhaps, better understand by a model ; 

 and it is possible to make one which, not only in form but in 

 principle, corresponds to the living mechanism it is intended to 

 illustrate. In the model the axial bundle is represented by a 

 strip of leather, the innumerable cells of the excitable cushion by 

 an india rubber bag. By a pump we are able to fill this cell or 

 cushion more or less with fluid, and thus to vary its tension, and 

 you see that if we increase the tension, the stem rises. By 

 diminution it suddenly falls, just as the .Mimosa leaf does when 

 irritated. 



We have come then to this point — that the reason why the 

 leaf suddenly [sinks on excitation is that the cells undergo a 

 sudden diminution of tension or expansion. But our inquiry is 

 not yet terminated. We have still to ask, How is this loss of 

 tension effected? The answer is, by discharge of water. In the 

 unexcited state all these cells are distended or charged with 

 liquid. Suddenly, when the structure is excited, they let out or 

 discharge that liquid, and it finds its way first into the inter- 

 cellular air spice , and secondly, out of the motor organ alto- 

 gether. This n e know to be a fact by an experiment of Pfcffer's, 

 which must be regarded as one of the most impoitant relating to 

 the mechanism of plants that was ever made. He observed that 

 if the leafstalk is cut off from the motor organ, a drop of fluid 

 appears at the cut surface at the moment that the latter bends 

 downwards on excitation, and that in the experiment described 

 just now, in which the upper part of the motor organ is cut off, 

 there is also, so to speak, a sweating of liquid from the cut 

 surface. 



We arc therefore certain that liquid escapes, but why does it 

 escape ? That I shall explain further on, and will now proceed 

 to two other examples. One is a plant which is a srreat favourite 

 in London, for it is one which flourishes even in London smoke 

 — Mimulus. For our purpose it is good chiefly because its 

 structure is very simple. It is one of those examples in which 

 excitability is associated with the function of fertilisation, and 

 inasmuch as this is a very transitory purpose, the property itself 

 is transitory. When the cells of the stigmalic surface are touched 

 they discharge their liquid contents, and consequent 

 limp. The outer layer of the Iid is elastic, and tends to bend 

 inwards. Consequently when the inner cells lose their elastic 

 resilience it is able to act, and the lip hends inwards. In another 

 allied plant. GoUfussia anisophylla (Fig. 3), which was described 

 forty years ago by the Belgi n naturalist Morren, we have the same 

 mechanism. In this plant, as shown in the drawing, the style 

 is not lipped but awd-shaped. It reaches to the mouth of tie 

 showy, orange-coloured corolla, to the inside of which it is 

 united by its under surface. It has a smooth side, the epidermis 

 of which is made up of numerous small prismatic cells, a'rd is 

 very elastic, and in the unexcited state concave, and a papillated 

 side beset with the nipple-like ends of cylindrical cells, which, 

 when unexcited, are distended with liquid. These cylindrical 

 cells are continuous with those of the conducting tissue of the 

 style. When an insect enters the flower, it does two things : it 

 charge.; the fringe of hairs on the inside of the corolla with 

 pollen, and touches the siyle, which, in consequence, bends 

 suddenly in the opposite di-ection to that in which it was bent 

 before, so as to plunge its stigmatic surface into the fringe. In 

 this motion the epidermis acts as a spring simply. So long as 

 the stigmatic tissue is turgid it cannot act. The moment its cells 

 lose their tension, off it goes. 1 



Another plant investigated by Morren is one of very different 

 organisation, but is one in which the existence of excitability 

 has an equally plain teleological interpretation. 1 

 Robert Brown, to whom plant-lore owes so much, when ex- 

 ploring the flora of Botany Bay, became acquainted with the 

 now well-known Australian plant called Stylidium. 5 [A specimen 

 from the Royal Gardens at Kew was exhibited.] Here is the 

 plant ( Fig. 4). The flower is too small to be easily seen, but the 

 diagram will enable you to understand the mechanism. It has 



1 'RJcherches sur le mouvement, &c, du style du Goldfussia aniso- 

 phylla." Mint, de 1' Acad. Kvyalc de Bruxelks. 1830. vol. xii. 



- Morren, " Recherches sur le mouvement et l'anatomie du Stylidium 

 gtaminifoliitm." Mem. de PAcad de Bruxeltes. t. xi., 1838. 



again to do with insects and fertilisation. In Stylidium the 

 anthers and stigma are united together at the summit of a 

 cylindrical stem which may be compared with the motor organ 

 of Mimulus. You might naturally suppose that they were 

 arranged so in order that the pollen from these anthers should be 

 at once received by the adjoining stigmatic surface. That it is 

 not so is evident from the order of development cf the flower, 

 for you find that at the moment that the anthers burst the stigma 

 is not yet mature. Consequently the pollen is not intended for 

 it, but for flowers which have come to maturity earlier, and the 

 mechanism which now interests us fulfils this purpose. The 

 figure shows the singular form of this strange flower. You 

 observe that the column, as it is called, is bent down over the 





Fig. 3.— Style, stamens, and part of corolla of Golofussia. In the left-hand 

 figure the style is in the uaexctted state, and is curved upwards, so that 

 the stigmatic surface looks towards the mouth of the corolla. On exci- 

 tation it suddenly assumes the position sho*n in the second figure, the 

 stigma looking towards the ro jts of the collecting hairs. 



Fig 4. —Flower of Stylidium, shoeing the column in the unexcited state, 

 terminating in the anthers and stigma, which are surrounded by con- 

 spicuous hairs. It is bent down at the mouth of the corolla, the four 

 principal lobes of which are seen, two on each side, and partly conceals 

 the fifth lobe or labellum. 



Fig. 5. — A floret of Centaurea as prepared for projection on the screen. The 

 corolla (() has been cut away so as to expose the five filaments (£), beset 

 with hairs, and united above into the anther tube (a). The filaments are 

 arched outwards, as in the unexcited state. 



corolla so as to be in contact with the odd-looking labellum, 

 which here takes the place of one of the petals. At the moment 

 that the anthers burst the column attains its greatest sensitive- 

 ness. 'I he slightest touch causes it to spring up, straighten itself 

 suddenly, and then bend over to the opposite side. The mecha- 

 nism resembles that of Mimosa and of Mimulus. There is a 

 spring, the action of which is restrained by the resilience of cells 

 distended with liquid. Suddenly these cells discharge their 

 contents, and the spring acts. 



And now let me pass to another group of plants which may 

 serve as a contrast to Stylidium. Stylidium may be ctlled an 



