VEGETABLE.] PHYSIOLOGY 63 neglected. The growth in length of a filament of Vauchcrin may be taken as a case illustrative of the importance of turgidity as a condition of growth in unicellular organs. This growth cannot be attributed to a greater hydrostatic pressure at the apex of the cell, for the pressure is necessarily the same at all points, and clearly it cannot be referred to a diminished resistance on the part of the primordial utricle to the passage outwards of water at that point. It must be referred to the cell-wall, and it can only be accounted for on the assumption that the elasticity of the cell-wall is less at the apex than at any other point of its surface. Growth is not, however, to be regarded as the result merely of the mechanical expansion of the cell which is rendered permanent. There is every reason to believe that the protoplasm takes an active part in producing this expansion, and in determining the direction in which expansion shall more particularly take place. The arrest of growth by strong light is a case in point. There is no reason to believe that this is to be ascribed to an increased rigidity of the cell-wall, or to a diminution of the attraction of the cell-sap for water. It can only be ascribed to a molecular change in the proto plasm, which causes it to offer considerable resistance to any change of form, either spontaneous or such as is induced by the hydro static pressure. The arrest of growth which, as we have seen, usually occurs when leaves are kept in continuous darkness is another case in point. The arrest of growth of the cells under these circumstances cannot be referred to a change in the physical pro perties of either the cell-wall or the cell-sap, but must be. attri buted to a change in the molecular condition of the protoplasm. The phenomena of heterauxesis, spontaneous and induced, have now to be considered. It will be convenient to deal with induced heterauxesis first, and we will begin with the case of a unicellular organ. It has been mentioned that heterauxesis, in the form of curvature, is induced by the action of light, gravity, &c. In asmuch as the hydrostatic pressure is necessarily the same at all points of the internal surface of the cell, the curvature must depend upon a local variation of the properties either of the cell- wall or of the protoplasm. In the case of the cell-wall either its rigidity is increased on one side, the concave, or its extensibility increased on the other, the convex. It is just conceivable, with regard to the action of light, that such a difference in properties might be induced by the more direct exposure of one side of a deli cate filament to light, though the difference of intensity on the two sides would be very small. But it is not at all conceivable that such a difference could be induced by the action of gravity, and no explanation can be regarded as satisfactory which fails to meet all cases of curvature. The cause of the curvature is doubtless to be sought in the protoplasm. Unilateral illumination of the organ, or an abnormal relation to the line of action of gravity, acts as a stimulus on the organ and causes an alteration in the properties of its protoplasm, which is perhaps of such a nature that it becomes relatively rigid on the side which becomes concave. The induced heterauxesis of nmlticellular organs is certainly of essentially the same nature as that erf unicellular organs. Applying the above explanation of the curvature of unicellular organs to nmlticellular organs, the conclusion to be drawn would be that the curvature of the latter is due to the induction of the same changes in the protoplasm in each of their growing cells. The phenomenon of spontaneous heterauxesis, as exhibited in nutation, may be accounted for in precisely the same way, but it is possible to imagine that it may be due to some extent in the case of unicellular organs to local variations in the extensibility of the cell-wall, and in that of nmlticellular organs to variations in the extensibility of the cell-walls of groups of cells on different sides of the organ. The phenomenon exhibited by mature mobile organs, such as the Function leaves of the Sensitive Plant, &c., remains finally to be considered, of pul- The movement of the leaf as a whole is effected by a group of cells, vimis. constituting a swelling, the jmlvinus, at the insertion of the main petiole, and of each leaflet by a similar organ at its attachment to the main petiole. The structiire of the pulvinus is briefly a mass of parenchymatous cells having the same structure as that described above, traversed by a strand of flexible fibro-vascular tissue. When the leaf is fully expanded, its position is maintained by an equality between the downward pressure of the portion of the pulvinus above the fibro-vascular strand and the upward pressure of the portion below it. The downward movement of the leaf as a whole is due to a sudden diminution of the upward pressure of the lower portion of the main pulvinus ; similarly, the upward movement of a leaflet is due to the sudden diminution of the downward pressure of the upper portion of its pulvinus. In both cases the diminution of pressure is due to a loss of turgidity of the portion of the pulvinus concerned : the cells become flaccid. This loss of turgidity has been shown to be due to an escape of water from the cells, which can only be accounted for by ascribing it to a change in the molecular condition of their protoplasm. In spontaneous movements this change is induced automatically, in induced movements by the action of a stimulus. This molecular change is probably of such a kind that the protoplasm takes up water into itself, and at the same time allows it to pass through. The recovery of turgidity is slow. The arrest of movement which is induced by long-continued darkness or by exposure to light is probably due to the prevention of the occurrence of molecular change in the protoplasm. The con duction of a stimulus, which undoubtedly takes place in the leaves of the Sensitive Plant, and probably in many other plant-organs (see above on heliotropism, geotropism, hydrotropism, tendrils), is effected by means of the delicate filaments of protoplasm which, as Gardiner has clearly shown in the pulvinus, are continuous between the protoplasm-bodies of adjacent cells. For the reproduction of plants, see RKPHODUCTIOX. Literature. Plie following works on the physiology of plants may be con sulted, Sachs, J.elirbuch (2d Eng. ed., Oxford, 1882) and Vorlesvngen iiber Pflamenphysiologie (Leipsic, 1882); Pfefter, PHanzenphysiologie (Leipsic, 1881); Van Tieghein, Traite de Botaniqne (Paris, 1884); Darwin, Climbing I lm>ts(S~5) and The Power of Movement in Plants (1SSO). (8. H. V.) INDEX TO PHYSIOLOGY. Absorption, of gases, 45. law of, 45. ,, of water, 44. Accelerating actions, 29. Alkaloids, 53. Amides, 54. Anabolism, 13, 19, 22, 49. Anaerobiotic plants, 51. Anastates. 19, 20. Animal spirits, 10. Aromatic substances, 52. Ash of plants, 45, 49. Basal ganglia, 37. Bitter principles, 53. Blood-vessels, influence of nerves on, 30. Brain, development of, 32, 33. ,, physiological anatomy of, 31, 32. ,, size and weight of, 33. Cane-sugar, 54. Carbon dioxide, 45. Cell-contents, 11. Cell-membrane, 11. Cells, functions of in plants, 44. ,, structure of, 44. Cell-sap, 44. Cell-theory, 11. Cell-walls, 43, 63. Central nervous organs, 20, 27. Cerebellum, 32, 38. Cerebral hemispheres, physiological anatomy of, 39. ., ,, removal of, 40. peduncles, 37. ,, vesicles, 31. Cerebrum, commissural fibres of, 40. ,, longitudinal fibres of, 40. ,, peduncular fibres of, 39. Chlorophyll, 48, 52, 53. Circulate vernation, 58. Circulation, in brain, 42. ,, osmotic, 46. Colouring matters in plants, 52. Consciousness, 20, 41. Contact in plants, 60. Contractility of protoplasm, 13, 62. Corpora quadrigemina, 32, 37. ,, striata, 31, 37. Corpus callosum, 31. Cranial nerves, 36, 42. Darwinian curvature, 60. Depressor nerve, 30. Ectoderm cells, 14, 15. Electrical stimulation of cerebral hemi spheres, 40. ,, of nerves, 24 sq. Electricity in plants, 56. Electrotonus, 24, 25. Encephalon, 32. Endoderm cells, 14, 15. Energy, accumulation of in plants, 56. ,, dissipation of in plants, 56. ,, source of bodily, 9. supply of in plants, 55 sq. Epinasty, 58, 60. Equilibrium, sense of, 3S, 39. Excitability of grey matter, 28. ,, of nerves, 24. Excretion, glandular, 53. ,, nectary, 53. Experimental method, 23. Experiments, on cerebellum, 38. ,, on cerebrum, 40. ,, electrical, 24, 25, 26, 41. Fats in plants, 54. Fatty bodies, 53. Fermentation, 51. Ferments, unorganized, 50. Ferrier s experiments, 41. Fixed-light position, 58. Food, 9. ,, of plants, 48, 49. Function, 10. Galvanotropism, 60. Geotropism, 59. Glands, influence of nerves on, 30. Glucose, 54. Goltz s experiments, 41. Gravity, influence on growth of plants, 58, 59. Grey matter of nerves, 28, 35. ,, arrangement and structure of, 40. Growth of plants, 57 si/. ,, ,, direction of, 5S. ,, rate of, 57. Heart, nervous arrangement of, 29. Heat, in nerves, 26. ,, source of plant-energy, 56. Heliotropism, 58, 59. Bering s speculations, 22. Heterauxesis, 58, 63. Hydrotropism. 60. Hyponasty, 58, 60. Inhibition, 22, 29, 35. Internal capsule, 37. Irritability, of nerves, 24. of protoplasm, 13. Katabolism, 13, 19 sq., 50. Katastates, 19, 20. Laticiferons tissue and cells, 48. Light, influence on growth of plants, 57. ,, ,, movement of plants. 62. ,, ,, transpiration, 46. source of plant-energy, 55, 56. Medulla oblongata, 32, 35. ,, ,, as conductor of ner vous impressions, 36. ,, ,, nuclei of, 36. ,, physiological ana tomy of, 36. ,, ,, as reflex centre, 36. Mesostates, 19. Metabolism, constructive, 13, 49. ,, destructive, 13, 50. Molecular actions, 18. ,, changes in muscle, 19. ., ,, nervous sub stance, 19. Motor areas, 41. ., centres in brain-cortex, 41. ., nerves, 8, 24. Movement, of body, 8. ,, co-ordination of, 38. ,, in plants, 56, 57 </., 62. protoplasmic, 13, 61. ,, of variation, 61. Muscles, contraction of, 8, 24 sq. Negative pressure, 47. Nerve-cells, 27. Nerve-currents, nature of, 25, 26. ,, velocity of, 25, 26. Nerves, chemical constitution of, 24, 28. classification of, 27. conductivity of, 25. ,, electrical phenomena of, 26. ,, nutrition of, 26. origin of in medulla, 36. stimuli of, 24. structure of, 23. Nervous centres, 28, 30. ., classification of, 30. impulses, 8, 20. ,, system, 8, 23 sq., 30 sq. ,, ,, comparative view of, in invertebrates, 30, 31. ,, ,, comparative view of, in vertebrates, 31, 32. tissue, 28. Nodus cursorius, 38. Nucleus caudatus, 37. ,, of cell, 11. lenticularis, 37. Nutation, 58.
Optic thalami, 31, 37.