648 



NATURE 



[July 22, 1920 



an ion with a double positive charge, the CO3 

 group an ion with a double negative charge. 

 These ions group themselves in the same way in 

 the calcite and potassium chloride structures, as 

 the models show, except that the form of the CO3 

 group distorts the cube into a rhombohedron. 

 The electro-negative atoms of carbon and oxygen 

 hold electrons in common, and form a closely 

 knitted group, and from their distance apart we 

 can form an estimate of the dimensions of the 

 outer electron shell ; it is the lower limit to which 

 the diameters tend at the end of each period in 

 Fig. z. 



In this an explanation is found of the large 

 diameters assigned to the electro-positive 

 elements, and the small diameters assigned to 

 the electro-negative elements, in Fig. 2. The 

 electro-positive atoms never share electrons with 

 their neighbours; they are therefore isolated in 

 the crystal structure, and appear to occupy a 

 large domain. The electro-negative elements, 

 bound together by common electrons, have to 

 be represented by small spheres. 



Comparing two crystals sych as sodium fluoride 

 and magnesium oxide, which have identical struc- 

 tures, we see that both may be represented by 

 alternate electron groups of the Neon type. In 

 the case of magnesium oxide the ions carry a 

 charge twice as great as the sodium and fluorine 

 ions, and the consequence is that the MgO struc- 

 ture, though identical in form with the NaF struc- 

 ture, has its dimensions reduced. The side of 

 the elementary cube has a length of 422 x lo-^ cm. 

 in the case of MgO, a length of 478 x lo-^ cm. in 

 the case of NaF. 



In diamond every carbon atom is surrounded 

 symmetrically by four other carbon atoms ar- 

 ranged at the corners of a tetrahedron. The 

 carbon atom has four electrons in its outer shell, 

 and, in order to complete the number eight re- 

 quired for stability, it shares a pair of electrons 

 with each neighbouring atom. The whole crystal 

 is thus one continuous molecule, and the great 

 hardness and density receive a simple explana- 

 tion. 



A crystal of an electro-positive element cannot 

 be bound together by common electrons. Here 

 we must suppose that the crystal consists of ions 

 and electrons, the ions representing the stable 

 electron systems, and the electrons being present 

 in sufficient numbers to make the whole assem- 

 blage electrically neutral. From the fact that 

 all crystals of electro-positive elements are con- 

 ductors of electricity we deduce that the electrons 

 have no fixed place in the system ; they move 

 under the influence of an electromotive force. 



It has been possible only to indicate the manner 

 in which crystal structure helps to elucidate the 

 structure of the atom, and many generalisations 

 have been made to which there are exceptions. It 

 is hoped that this discussion will show the in- 

 terest of the study of crystals. In a crystal there 

 are countless atomic groupings oriented with per- 

 fect regularity. Individually their effect is too 

 small to observe, but by illuminating the crystal 

 with X-rays, the wave-length of which is much 

 less than the distance separating the atoms, we 

 can make use of their concerted effect on the rays 

 to enable us to see into the intimate structure of 

 matter. 



Researches on Growth of Plants.^ 



By Sir Jagadis Chunder Bose, F.R.S. 



The General Principle Determining Tropic 

 Movements. 



THE movements in plants under the stimuli of 

 the environment — the twining of tendrils, the 

 effect of temperature variation, the action of fight 

 inducing movements sometimes towards and at 

 other times away from the stimulus, the diametric- 

 ally opposite responses of the shoot and the root 

 to the same stimulus of gravity, the night and day 

 positions of organs of plants^ — present such 

 diversities that it must have appeared hopeless to 

 endeavour to discover any fundamental reaction 

 appUcable in all cases. It has, therefore, been 

 customary to assume different sensibilities espe- 

 cially evolved for the advantage of the plant. But 

 teleological argument and the use of descriptive 

 phrases, like positive and negative tropism, offer 

 no real explanation of the phenomena. I propose 

 to describe experimental results from which it will 



1 Continued from p. 6iy. 



NO. 2647, "VOL. 105] 



be possible to discover an underlying law which 

 determines the various tropic movements in plants. 

 Direct Effect of Stimulus. — In the motile pul- 

 vinus of Mimosa the excitation caused by stimulus 

 causes a sudden diminution of , turgor and con- 

 traction of the cells. With regard to this fall of 

 turgor it is not definitely known whether excita- 

 tion causes a sudden diminution in the osmotic 

 strength of cell sap or increase in the permeability 

 of the ectoplast. The state of excitation in a 

 vegetable tissue may, however, be detected, as I 

 have shown elsewhere, by the following indica- 

 tions : (i) diminution of turgor; (2) contraction 

 and fall of leaf of Mimosa ; (3) electromotive 

 change of galvanometric negativity ; (4) variation 

 of electric resistance ; and (5) retardation of the 

 rate of growth. 



Contimiity of Physiological Reaction in Growing 

 and Non-growing Organs. 



In investigations on the effect of all modes 

 of stimulation, mechanical, electrical, or radia- 



