466 



NA TURE 



[March 15, 1894 



the sensory nerves. Thus the peripheral "plume" of the 

 pyramidal cells would be the fpot at which the voluntary motor 

 impulse arises, to be communicated to the body of the pyramidal 

 cell, and ;o to the fibres forming the pjramidal tract. 



When an electrical stimulus is applied to the cortex, 

 muscular movements are produced, because the stimulus acts 

 either u[ on the " plumes " or u] on the nervous fibrils whose 

 function it is to carry impulses to the "plumes." Every nerve 

 centre is made up of four constituents : nerve cells with short 

 axis cylinders, terminal nerve fibres coming from other centres 

 or from distant parts of the same centre, nerve cells with long 

 axis cylinders, and collaterals which arise from axis cylinder 

 prolongations of cells, or from nerve fibres of the whole sub- 

 stance. In the retina, olfactory bulb, and molecular layer of 

 the cerebrum, there are in addition cells characterised by the 

 absence of differentiation of nervous and protoplasmic ex- 

 pansions. 



In organs where it is well established that excitatory pro- 

 cesses arise the cells are polaiised, i.e., the nervous impulse 

 always enters by way of the protoplasmic apparatus, or by the 

 body of the cell, and leaves by the axis cylinder, which trans- 

 mits it to a new protoplasmic apparatus. The difftrerjtiation 

 of the protoplasmic apparatus is for the purpose of enabling 

 each cell to be connected with different kinds of nerve fibres, 

 and the more varied the protoplasmic expansion, the greater 

 the number of cells under whose influence it comes. In the 

 same way the more the nervous expansion of a cell is exttnded, 

 and the more collaterals it possesses, the greater is the number 

 of cells to which its impulses may pass. 



In the pyramidal cell of the brain of mammals, the differen- 

 tiation and extension of the protoplasmic expansion, and the 

 multiplication ot the collateral and terminal nervous twigs are 

 carried to their highest point, and on descending the scale both 

 the differentiation and the number of twigs becomes rapidly less ; 

 in fish the pyramidal cell is absent. 



As regards the education of the brain mental activity is not 

 able to improve the cerebral apparatus by augmenting the 

 number of cells, as the nervous elements lose their power of 

 dividing during the embryonic period, but it is probable that 

 intellectual exercise may produce in certain regions of the brain 

 a large development of the protoplasmic apparatus and of the 

 system of nervous collaterals, so that the associations already 

 existing between certain groups of nerve-cells would be perfected 

 by a further development of terminal twigs, of protoplasmic 

 endings, and of nervous collateral branches, whilst quite new 

 intercellular connections might be established by a new form- 

 ation of collaterals and protoplasmic expansions. 



" Vis a vis de la thcone des rcseaux celle des arborisations 

 libres des expansions cellulaires susceptibles de s'accroitre 

 apparait non seulement comme plus probable, mais aussi comme 

 plus encourageante. Un leseau continu pre irtabli — sorte de 

 grillage de fils lelt-graphiques oii ne peuvent se crcer ni de nou- 

 velles stations ni de nouvelles lignes — est quelque chose de 

 rigide, d'lmmuahle, d'immo.lifiable, qui heurte le sentiment que 

 nous avons tous que I'organe de la pentee est, dans certaines 

 limites, malleable et susceptible de perfection, surtout durant 

 I'cpoque de son developpement, au moyen d'une gymnastique 

 mentale bien dirigce. Si nous ne craignions pas d'abuser des 

 comparaisons, nous dcfendrions notre conception en disant que 

 'ecorce cerebrale est pareille a un jardin peuple d'arbres 

 nnombrables, les cellules pyramidales, qui, giaceii une culture 

 ntelligente, peuvent multiplier leurs branches, enfoncer plus 

 oin leurs racines, et produire des fleurs et des fruits chaque fois 

 plus varies et exquis. 



" Du resle nous sommestres lo^n de croireque I'hypothese que 

 nous venons d'esquisser puiss-e a eile seule expliquer les grandes 

 difierences quantitatives et qualitatives que presente le travail 

 ccicbral chez les divers animaux et danslameme especeanimale. 

 La morphologie de la cellule pyramidale n'est qu'une des con- 

 ditions anatomiques de la pen^ee. Or cette morphologie speciale 

 ne suffiia jamais a nous expliquer les enorm.es differences qui 

 existent au point de vue fonctionnel entre la cellule pyramidale 

 d'un lapin et celle d'un homme, ainsi qu'entre la cellule pyra- 

 midale de I'ecorce ceiebrale et le corpuscle L-toik- de la moelle 

 ou du grande sympalhique. Aussi a notre avis est-il tres pro- 

 bable qu'en outre de la complexite de leurs rapports les cellules 

 pyramidales possedent encore une structure intraprotoplasmique 

 toute speciale, et meme perfectionr.ee dans les intelligences 

 d'clite, structure qui n'existerait pas dans les corpuscles de la 

 moelle ou des ganglions." 



NO. 1272, VOL. 49] 



ON THE IRRITABILITY OF PLANTS.^ 



C OME years ago I published my observations on the strange and 

 , till then undescribed effect produced by various bodies on 

 the sporangiferous hyphse of I hycomyces iiitens, well known to 

 every plant-physiologist. To be brief, the phenc menon consisted 

 in the fact that certain bodies attract Phycoviyccs, i.e. these 

 bodies cause the hyphae growing in their vicinity, at a distance 

 of from one to two centimetres, to make curves in their growth, 

 the concavity of which is directed tov\ards the said body. This 

 was particularly the case with iron ; zinc and aluminium 

 exhibited the same phenomenon, though in a smaller degree 

 (aluminium only so slightly, that I now feel inclined to count 

 this body among the inactive ones), while other metals showed 

 no effect. In many other bodies the same effect was observed. 

 The sporangiferous hyph:e, on the other hand, have a repellent 

 effect on each other. I formerly designated this phenomenon 

 as dependent on " physioh^gical action at a distance." 



At the Edinburgh meeting of the British Association for the 

 Advancement of Science, held in August, 1892, Prof. L. 

 Errera, of Brussels, read a paper on this subject, which was 

 published in the Report of the Society, p. 746, having appeared 

 earlier in the " Annals of Botany " (vol. vi. No. 24, December, 

 1892). lie considered the phenomenon to depend on a kind of 

 hydrotropism. 



It is a well known fact that the sporangiferous hyphje are 

 negatively hydrotropic, i.e. that they curve away from a surface 

 which discharges aqueous vapour, and the reciprocal repulsion 

 of the hyphse was considered by Errera to be a case of negative 

 hydrotropism. From this it was naturally concluded that they 

 are, on the other hand, attracted by a body that absorbs water. 

 The effect of iron, since iron does actually absorb water in a 

 damp atmosphere, is set down by Errera as a confirmation of 

 this supposition. Even in other bodies which absorb water, 

 Errera was able to find the same effect of attraction ; indeed, 

 in one case the inflexion of the hyphse led to the discovery of 

 the hygroscopicity of certain bodies. Thus the phenomenon 

 would be bereft of its mysterious character, and classified among 

 the already known qualities of this plant. 



According to my experience, however, the explanation ot 

 Errera is not sufficiently well based to be yet admitted. 



If iron acts as a hygroscopic {sit venia verba) body, we should 

 expect the phenomenon to be very clearly ohseivable in these 

 bodies, which are known to be particularly hygroscopic ; for in- 

 stance, potash and calcium chloride. Butif astick of caustic potash 

 is fixed in the usual way above the culture of Phycomyces, taking 

 care that the fluid dripping from the stick does not fall on the 

 hyphse or on the substratum, but into a small glass tube closed at 

 the bottom, no attraction will be observed. The stick of potash 

 I absorbs much water from the atmosphere, its upper layers 

 ' actually deliquesce, but, neither in its vicinity nor at a distance, 

 1 do the hyphae undergo any regular deviation from their direction 

 1 of growth. I have made this experiment several times, and 

 ; always with the same negative result. It is the same with 

 soda. With solid calcium chloride it is difficult to work, be- 

 I cause it deliquesces too quickly. I therefore used a solution of 

 calcium chloride (one part of salt to one and a half part of 

 1 water), with which I soaked a dry cylinder of plaster. This 

 I solution slowly absortied aqueous vapour from the air; the 

 j olinder consequently acted as a hygroscopic body, but no at- 

 traction could be observed. In one experiment the increasing 

 weight of the cylinder (length 50 mm., diameter il mm., 

 weight 4*904 gr. ) was observed duiing the experiment; it 

 amounted in four hours too'262gr. , and even then the body was 

 not yet saturated with aqueous vapour. 



Dry plaster also actively absorbs water from the air. I took 



a slab, measuring 80 x 35 x 10 mm , and dried it at ico''; 



it weighed 23 '077 gr. During an experiment of six hours this 



slab was without effect on the Phycomyces ; but in that time it 



had comiensed l '665 gr. of water. Now we might suppose that 



in this case the slab, by absorbing so much water, very soon 



came into a state in which it caused, neither positively nor 



negatively, hydrotropical curvatures ; that in fact it had absorbed 



too much water to effect attraction, and too little to cause re- 



; pulsion. But in the following six hours it still increased! 



■ 0049 gr. in weight, without exercising even now the least effect. 



' on the fungi. 



In comparison with thi.'^, a plate of iron absorbs very little- 

 ! water. Such a plate, the total surface of which was 4950 mm.^ 



1 "Ofversigt af Finsk. Vet. Soc. Furhand." H;ifl xxxvi. 1894 



