Browniaa movement of the particles {see Bayliss, Principle of General Physiology, 1918). On the basis of the colloid natur of the plasma membrane many of the phenomena of cell per meability may be explained. On the colloid theory of protoplasm the living organism has been defined as " a specific complex o dynamic changes occurring in a specific colloid substratum which is itself a product of such changes and which influences their course and character and is altered by them " (Child Senescence and Rejuvenescence, 1915).
Further investigation of the fundamental process of carbondioxide assimilation has confirmed the work of F. F. Blackman and his school, which showed that the rate of the process is controlled mainly by temperature, light intensity and concentration of carbondioxide Any one of these factors may control the rate of the process and so act as a " limiting factory." The amount of chlorophyll, since i( controls to a large extent the amount of light absorbed, should also be a controlling factor, and Willstatter and Stoll (Untersuchungen Tiber die Assimilation der Kohlensaure, 1918) have shown .that this is so. With the help of the methods of extracting and estimating the leaf pigments developed earlier by Willstatter they have been able to relate chlorophyll content with rate of assimilation. Their observations have brought out the interesting fact of the importance of some unknown factor (possibly of enzymic nature) which may be termed the protoplasmic factor. The existence of this factor is demonstrated by the observation that, relative to the amount of chlorophyll it contains, the assimilating activity of a yellowing leaf may be many times that of a green leaf. The existence of some such factor is also demonstrated by the observations of Miss Irving (Annals of Botany, 24, 805, 1910) and Briggs (Proc. Roy. Spc. B., 91, 249, 1920), who demonstrated that during the greening of etiolated leaves chlorophyll appears some time before the process of assimilation begins. Osterhout (Jour. General Physiol., 1919) and Warburg (Biochem. Zeit., 100, 1919) have confirmed the high temperature coefficient of the process of carbon assimilation which was first demonstrated by Matthaei. This high temperature coefficient shows clearly that the process of carbon assimilation is not solely a photochemical process but is linked with one or more " dark " reactions. Warburg has also been able to show that light which was intermitted 16,000 times a minute the light and dark periods being of equal length caused as much assimilation as con- tinuous light of the same intensity. Light received intermittently by the plant was thus twice as effective as that received during continuous illumination. Reference must also be made to a work of great value, Jorgensen and Stiles' critical review of investigations of carbon assimilation up to the year 1917 (Jorgensen and Stiles, Carbon Assimilation, 1917; originally published in the New Phytol- ogist,_ 1916-7). An important method of estimating assimilatory activity under natural conditions by following the growth of the leaf area and of the dry weight of the whole plant was first used by Gregory (Report Exper. & Res. Sta. Cheshunt, 1918), and has been employed by Briggs, Kidd and West (Annals of Applied Biology VII., 1920). The permeability of the living cell is another aspect of plant physiology which has received much attention ; but although many observers have studied the rate of entry or exit of substances, it cannot be said that great progress has been made in elucidating the mechanism of absorption, accumulation and translocation. Measurements of electrical conductivity have been largely used for estimating the rate of passage of electrolytes in or out of the cell, and methods based on the rate of deplasmolysis have also been employed. Stiles and Kidd (Proc. Roy. Soc. B., 90, 1919) using carrot slices and the conductivity method, have estimated carefully the rate of the entry of the cations and anions of a number of simple salts, the rate of entry being apparently related to the mobility of the ions. They have confirmed (using, however, a more satisfactory method) the work of Nathansohn on the balance between the concentration of salts inside and outside the cell. They show that with weak external solutions there is a very marked " heaping up " of material in the cell. For example, at equilibrium the concentra- tion of potassium chloride in the cell may be 25 times that of the external solution. The mechanism of accumulation is obscure, but the equilibrium appears to follow the adsorption law. That marked and repeated changes of permeability can occur in the living cell has been shown by Osterhout (Science, 35, 1912; Bot. Gazette, 59, 1915; and other papers), who used the method of measuring the effect of various salts on the electrical resistance of a pile of discs of the thallus of Laminaria. The electrical resistance is taken as a measure of the permeability to ions of the plasma membranes. Increases of 50% and decreases of 20% in the resistance could be sustained repeatedly without injury. Lepeschkin and Trondle have demonstrated that changes in permeability can be brought about by light, and so must be normal phenomena in the life of the cell ; and Blackman and Paine (Annals of Botany, 32, 1918), using the elec- trical conductivity method for determining the rate of exosmosis of electrolytes from the pulvinus of Mimosa, have been able to follow the change continuously and have demonstrated that although light increases and darkness decreases permeability, yet the sudden change from light to darkness causes a sudden small increase of
exosmosis which is soon, however, followed by a rapid decrease. The question of the meaning of the growth process and its analysis have engaged the attention of many workers on both the animal and vegetable side. Robertson especially has applied an auto- catalytic equation to express the growth period of an ordinary organ, but as shown by Enriques it is only one of a number of possible equations. Mitscherlich (numerous papers from 1909 onwards in Landw. Jahrb., Land. Versuchs-Stat.,etc.) has put forward an equa- tion to express the relation of crop yield to external factors, based on the supposition that the increase of the deficient factor is effective m proportion to the departure of the yield from the maximum yield obtainable. V. H. Blackman (Annals of Botany, 33, 1919) has laid stress on the fact that the plant increases on the continuous com- pound interest principle, since with increase of leaf area its capacity for assimilation increases, and this leads to a still further increase in the rate of assimilation. The rate at which dry material is added, assuming it to be added continuously, is termed the " efficiency index."
_ J. C. Bose has continued his investigations of the growth and irritability of plants (Researches on Irritability of Plants, 1913, and Transactions of the Bose Institute, 1918). He has devised a special instrument, the high magnification Crescograph, by means of which the elongation of plant organs can be magnified more than a million times. By this instrument not only can very minute contractions and expansions be observed but also changes which occur in as short a time as a fraction of a second. Bose has also investigated in great detail the various electrical responses to stimulation which plants exhibit. He has shown, for example, that when a stem which ' contains movable starch grains in its endodermis is placed hori- zontal, a marked difference of potential is to be observed between some neutral point (such as a leaf) and the interior of the stem. It was further demonstrated by the use of a probe, in the form of a fine insulated platinum point which could be forced into the tissue of the stem, that the difference of potential developed is highest when the point of the probe is in contact with the endodermis of one side of the stem, falls to a minimum when the probe reaches the centre of the stem, and rises again to a maximum (but with the direction reversed) when the probe reaches the endodermis at the other side of the stem. These observations provide additional evidence of the part played by the statoliths of the endodermis in the gcotropic response.
In the field of irritability the most important new point of view put forward is that of Blaauw (Med. Landouw., Wageningen, 15, 91, 1919) on the nature of the phototropic reaction. This investigator 5rst concentrated his attention on the effect of light as such, apart Tpm light direction. He made a very careful series of experiments with the sporangiophore of Pilobolus and the hypocotyl of Heli- anthus. By an ingenious arrangement of mirrors the plant was lluminated equally all round with electric light of various intensities. The rate of growth was measured every few minutes, and thus it was determined that the " light growth reaction," as Blaauw terms t, is a very complex effect. With continuous light Phycomyces shows a latent period of 3-9 min., and then the rate of growth begins to rise, reaching in 7-10 min. a maximum increase of 41-74%. The ncrease is followed by a fall and then several rises and falls follow,
- he normal rate of growth being finally reached, except with very
ligh intensities.^ A similar result is obtained with Helianthus, but
- he main effect is a reduction of growth instead of an increase as in
Phycomyces. Blaauw explains phototropic effects as quite in- dependent of light direction, holding that they are really due to
- he different intensity of the illumination of the two sides as de
Candolle maintained long ago. The fact that both Phycomyces and Helianthus show a positive phototropic reaction while the light growth reactions of the two are opposite in nature, is explained by the ens-like action of the glassy sporangiophore, which causes a higher ight intensity on the further side. Buder confirmed this explanation or he has shown that by placing the sporangiophores in paraffin oil, which abolishes the lens action, the response is reversed. The oot of Sinapis alba shows a negative light growth reaction and nega- ive phototropism, but here again, owing to the shape of the apex, he side away from the light is the more highly illuminated. Blaauw
- laims that plants have no mechanism for the perception of light
direction, and that there is no such thing as a real phototropic re- action, but only a light growth reaction. (V. H. B.)
III. Chemistry of the Sap Pigments of Plants. Flowers derive heir tints from two very different classes of coloured com- iounds, termed plastid and sap pigments respectively. The ormer include chlorophyll, carrotin, xanthophyll and allied ompounds, and are not soluble in water. Chlorophyll rarely nters into flower colourings, but compounds of the carrotin and canthophyll group are responsible for most of the bright yellow .nd orange flower colours, whilst in the presence of anthocy- ns they yield browns, bronzes, etc.
The sap pigments are water-soluble glucosides, and may in the lain be subdivided into two groups. One group, the flavone and avonol colours, contains compounds which, though usually present n the cell sap of flowers, rarely give rise to colour effects as they are
