December i. 1923] 



NA TURE 



799 



[ would give for the hydrogen atom a volume equal 

 to that of a sphere of radius 0-53 x 10 "^ cm. But the 

 normal hydrogen atom, as is now known from the 

 researches of Bohr, consists of two electric charges, 

 describing a circular orbit, one about the other, of 

 radius precisely equal to 0*53 xio"^ cm. As regards 

 collisions with other molecules, this invertebrate 

 structure, consisting of two point - charges with no 

 material connexion between them, appears to reserve 

 for itself a three-dimensional spherical volume with as 

 much precision as though it were a sphere of infinite 

 hardness. 



The explanation of this infinite hardness is to be 

 found in the intangible fetters of the quantum dynamics. 

 The nature of these fetters is not in the least under- 

 stood, but it is believed that they are such that no 

 force in creation can cause the electron of the hydrogen 

 atom to describe a smaller orbit than the normal orbit 

 of radius 0-53 x io"8 cm. If it is further supposed that 

 this orbit is free to assume all orientations in space we 



begin to understand why it is legitimate, for kinetic 

 theory purposes, to treat the hydrogen atom as an 

 infinitely hard sphere of radius 0-53x10"* cm. The 

 quantum theory brings us back, in a sense, to the 

 infinitely hard spherical atoms of Lucretius, and the 

 radius of these spherical atoms can now be calculated 

 with precision from the quantum theory ; their infinite 

 hardness is beautifully exemplified in the experiments 

 of Franck and Hertz. 



It is thus seen that the a and b of Van der Waals 

 admit of exact interpretation in terms of the physical 

 conceptions of to-day. His b arises from what we may 

 call the quantum forces — the perfectly unyielding 

 restraints which bind the electrons of an atom down to 

 definite orbits — while his a arises from the ordinary 

 electric field of force. It is the b of Van der Waals 

 which saves us from immediate annihilation, through 

 positive and negative charges rushing together to their 

 mutual destruction, just as it is his a which saves us 

 from rapid disintegration. 



The Nerves of Plants.^ 



By Prof. Henry H. Dixon, F.R.S. 



'X'HE general similarity of the distribution of the 

 J- fibro-vascular bundles in plants and that of 

 the nerves in animals was early noticed. These 

 structures in plants were in consequence often called 

 nerves. However, anatomists and physiologists alike 

 have long held the view that the likeness is merely 

 superficial, and is not based on any real physiological 

 or anatomical resemblance. 



In plants — as in animals — the receptive and respon- 

 sive regions are often quite distinct from one another, 

 and may be widely separated. What becomes of the 

 stimulus between the two, and how is it transmitted ? 

 Remarkable experiments during the last ten years 

 have given the answers to these questions. 



First may be summarised, in a few words, Ricca's 

 work on the sensitive plant. Mimosa. The phenomena 

 of transmission of stimuli in this plant are as striking 

 as they are well known. The stimulus is propagated 

 through its organs at velocities variously estimated at 

 10-20 mm. per sec. This speed is fast among plants, 

 but very slow when compared with the velocity of 

 transmission of stimuli along animal nerves. 



Two views were suggested to account for this pro- 

 pagation. The first referred the pas.sage of the 

 stimuli to those excessively fine strands of protoplasm 

 which, penetrating the walls of the living cells, place 

 the protoplasts of adjacent cells in communication 

 with one another. This view was a product of a period 

 obsessed with the physiological importance of these 

 then recently discovered protopla.smic fibrillae, which, 

 in all probability, have only a developmental signifi- 

 cance. These fibrillae composed of living matter were 

 supposed to convey stimuli just as the living processes 

 of the nerve cells do in the animal body. 



This view was soon rendered untenable when it was 

 shown that stimuli are effectively transmitted even 

 after the protoplasm of the cells of the transmitting 

 organs was killed by the application of heat. 



' Synopsis of a lecture delivered before the Royal Dublin Society on 

 November 9. 



NO. 2822, VOL. I 12] 



To meet this new growth of knowledge Haberlandt 

 developed his theory, that the stimuli are transmitted 

 in Mimosa in the form of a pulse in the water filling 

 certain elongated tubular cells situated in the bast of the 

 bundles. At the best this was an unsatisfactory theory. 

 For this method would require a much higher velocity of 

 transmission than is observed, and it was wellnigh im- 

 possible to imagine how the turgor requisite to transmit 

 this pulse could be maintained after the protoplasts of 

 these tubes had been rendered permeable by heat. 



In 1 9 14 Ricca gave the coup de grace to the pulse 

 theory. He showed that the stimulus is transmitted 

 through a strand of Mimosa wood from which all the 

 bast, including the tubes of supposed transmitting 

 function, had been removed for a considerable length. 

 By a series of beautiful experiments Ricca showed 

 that the wood, as Dutrochet long ago believed, trans- 

 mits the stimulus, and that it does this even when 

 all its living elements are eliminated. Further, he 

 demonstrated that the mechanism of the transport is 

 the transpiration current. This carries in its stream 

 a substance, or hormone, originating from the receptive 

 cells, to the cells of the reactive region and so evokes 

 their response. Ricca's work also disposes of a more 

 recent view that the stimulus is transmitted as an 

 electrical disturbance in the bast. 



Almost at the same time as Ricca was disposing of 

 the older views regarding the transmission of stimuli 

 in Mimosa, Boysen-Jensen was carrying out experi- 

 ments on the phototropic reactions of seedlings, which 

 were bound to have a profound effect on the received 

 views regarding the propagation of stimuli. 



When the tip of a grass-seedling is illuminated on 

 one side a stimulus is transmitted from the receptive 

 region downwards in the seedling and evokes a curva- 

 ture in the shaded part. Boysen-Jensen found that 

 this stimulus was transmitted downwards even when 

 the protoplasmic continuity of the cells of the receptive 

 apex with those of the responsive region was severed 

 by complete section. 



