The Physiology of the Ascent of Sap

LIFE MOVEMENTS IN PLANTS, Vols. III. and IV. With 124 Illustrations, 8vo. The ascent of sap has been the most elusive problem in Plant-physiology. The obscurity which has surrounded the subject has been in a great measure due to the lack of adequate means of detection and accurate measurement of the rate of ascent, of transpiration, of exudation, and their induced variations. Various types of automatic recorders of great sensitiveness and precision have been devised and are described in the present work, which have been of signal service in the investigations of which an account is here given.

The result of these researches is to prove the existence of active pulsating cells throughout the length of the plant, in and from the absorbing root to the transpiring leaf. It is the pumping action of these cells that gives rise to the physiological conduction of sap, even in the absence of root-pressure and transpiration ; it also injects liquid into the xylem, setting up an intra-vascular pressure with the consequent mechanical transport of fluid.

The situation of the active cells has been localised by means of the Electric Probe ; the cellular pulsations con- cerned in the ascent of sap have been recorded by an automatic method. The invisible changes in the interior of the plant have thus been revealed, and the effect of the changes of the environment determined from the responsive variations in the pulse-record. Other investigations are described which show that there are two distinct modes of inter-communication and inter-action between distant organs in plants: (i) the transfer of matter, and (2) the transmission of motion. The first

of these is brought about by the movement of sap, and the second by the excitatory nervous impulse. They give rise to two reflexes at a distance, the hydraulic reflex being antagonistic to the nervous reflex. There are, no doubt, many such reflexes corresponding to the various modes of stimulation. The complexity of the life-movements is, in fact, the expression of the combined effects of concordant and antagonistic reflexes. The ascertained facts justify the important generalisation of the unity of the physiological mechanism in plants and animals. Further investigation of the simpler life of plants may therefore be expected to lead to the solution of many intricate problems in animal life.

It affords me much gratificatioi) to associate this work with the ' Cossimbazar Endowment,' founded for my Insti- tute by the enlightened interest taken by the Maharajah Sir Manindra Chandra Nandy, K.C.S.I., of Cossimbazar, in the advancement of research. I also take this opportunity of acknowledging the very efficient help which has been rendered to me by my research- assistants and scholars. Physical and physiological theories — Inconclusive character of Stras- burger's poisoning and scalding experiments — Root-pressure

Rhythmic vegetable tissue — Autonomous pulsation in Desmodii.m gyrans — Multiple response under strong stimulus — Pulsations in growtli — Characteristics of pulsatory activity^ — Effect of varia- tion of internal hydrostatic pressure — Effect of maximal stimulus — Effect of sub-minimal stimulus — Modification of response in sub-tonic specimens — Effect of variation of temperature on rhythmic activity — Arrest of pulsation at the critical thermo- metric minimum — Effect of anaesthetics^ — Effect of dose — Action of poison — Tests for pulsatory activity — Summarj^

Detection and record of ascent of sap — Mechanical method of Erectile Response — The Automatic Recorder for erectile response — Erectile response of Mimosa, Chrysanthemum and Impatiens — The Osmotic Theory — Theory of suction and root-pressure — Ascent of sap in the absence of root-pressure and transpiration — Depressed rate of ascent under increasing drought — Ascent of sap in cut stems previously exposed to air — Function of the xylem — Summary ........

Difference of velocity of ascent in cut and rooted specimens — In- fluence of the previous history of the plant — The Duplex Method — The effect of drought — The effect of physiological anisotropy induced by stimulus — Determination of velocity in the reverse direction — Summary ...... The Potograph — -Effect of physiological agents in modification of ascent — Effect of diminished internal pressure — Effect of stimulus — Modifying influence of tonic condition — Effect of variation of temperature on ascent and on growth — The critical thermo- metric minimum — Drooping of leaves during frost — Phenomenon of accommodation — Effect of anaesthetics — Effect of poison — Method of exudation — Strasburger's experiments — Summary

Physical evaporation and physiological excretion — Isolation of absorbing, conducting, and excreting organs — The Bubbling Method for measurement of transpiration^Comparison of transpiring activity of different species of plants — Ratio of transpiration from upper and lower surfaces of leaves — Deter- mination of transpiration from a single stoma — Transpiration in the absence of evaporation — The role of evaporation — Physio- logical continuity in stem and leaf — Crucial tests of physiological activity underlying transpiration — Effect of variation of tempera- ture— Effects of sub minimal and maximal stimulus — Summary

The Micro -Transpirograph— Effect of diminution of turgor on trans- piration— Effect of stimulus — Opposite effects of stimulation of upper and lower surfaces of leaf — Effect of high frequency Tcsla current — Effect of electric waves — Effect of statical electric induction — Effect of thermal rays — Effect of light — Effect of red and of blue light — Effect of carbonic acid gas — Effect of ether — Effect of chloroform — Summary . . . . . . 98 Diurnal variation of transpiration in plants with roots — Diurnal variation after removal of the root — The Radiograph — Diurnal variation of temperature and of light^Balancing evaporation against transpiration — The Differential Balance — The optimum temperature for transpiration — Summaiy . • n?

The Recorder of Exudation — The Tilter and the Electromagnetic Writer — Composition of exuded sap — Continuous record of exudation — Effect of drought — Effect of mechanical and electrical stimulus — Effect of poison — Effect of anaesthetics — Continuity of action in root and in shoot — Activity of terminal layer at the cut end — Expulsion of sap by living cells — Summary . . . . . . . . . .131 General considerations — Diurnal periodicity of root-pressure — The recording apparatus — Relation between temperature and pressure — Diurnal variation of pressure in deciduous trees — Diurnal variation of exudation — The effect of hght — Summary . 146

Complexity arising from fluctuating factors of absorption and ex- cretion— Hydraulic and electric model — Diurnal variation of pressure in root stock — Effect of light — Explanation of irregular variation of pressure in trees — Diurnal variation of exudation in root stock — Positive and negative exudation — Exudation in Pithecolobium — Summary, . . . . . . .160 Exudation from the Mango-tree — Chemical analysis of the exuded sap — Period of maximum pressure — Absence of exudation from hole drilled into the tree — The existence of a cavity due to dis- integration of alburnum — The lateral injection of sap by active cortex — Enhanced secretion due to local rise of temperature — Summary . . . . . . . . . .170

The Indian Date Palm — The Palmyra Palm {Borassus flabellifer) — The maximum quantity of exudation in a season — The total yield of sugar — Diurnal variation of exudation in Phmtix sylvestris — Explanation of greater exudation at night— Diurnal variation of exudation in Palmyra Palm — The action of sunlight — Absence of root-pressure — Stimulus for initiation of exudation — The magnetic analogue of polar action of cells in absorption and excretion — Summary . . . . . . . .178

Electric variation with change of turgor — Electrometric determina- tion of velocity — Galvanometric determination — Shock- effect of the hydrostatic blow — Simultaneous determination of velocity of ascent by mechanical and electrical methods — Determination of velocity by the Di-phasic method — Summary . . .194 The Electric Probe for detection of pulsation in the interior of the plant — Turgor and electric variation during a single pulsation — Electric pulsation of Desmodiiim — Periodic groupings in pulsations — Record of pulsation of a single cell — Cellular pulsa- tion in herbaceous plants — Pulsating cells in trees — Pulsatory activity modified under variation of temperature — Record of pulsation by Einthovcn galvanometer — The period of a single pulsation — Summary ........ 206

Localisation of active layer of pulsating cells in Impatiens — Localisa- tion in Brassica — Amplitude of pulsation at different depths — Theory of the electric determination of wave-length — Successive electric maxima and minima — Determination of wave-length in Chrysanthentitm and in Musa — Change of wave-length under physiological variation — Upsetting of the phase-difference by passage of electric current — Summary . . . . .216 Initiation of pulsation — Effect of stimulus — Effect of differential hydrostatic pressure — Effect of constant electric current — Effect of variation of temperature on pulsation — Effect of ana?sthetics — Effect of diminished internal pressure — Summary . . . 232

Interaction between distant organs — Sachs's experiment of the growth of a branch inside a dark box — Hydraulic convection and nervous conduction — Importance of stimulus in maintenance of life-activity — The Leaf a catchment-basin for reception of stimulus — Stimulation of internal cortex by transmitted ex- citation of sunlight — Antagonistic action of the hydrauHc and nerve reflexes — Dual impulses under stimulus — Opposite effects of direct and indirect stimulus — Explanation of opposite geotropic responses in shoot and root — The co-ordination of nervous reflexes — Dia-heliotropic attitude of leaves — Summary 244

4. ,, Effectof Electric Stimulus on Pulsation of Z)es?worf2MW/ £4 8. ,, Effect of Poison on Pulsation of Z)es?«o^noM . . 22 g. Erection of Drooping Shoot on Application of Water at cut 10. Photographs of a Potted Specimen of Impatiens before and 11. Automatic Recorder for the Erectile Response of Drooping 13. Record of Erectile Response of Leaf of Mzwosfl on Irrigation . 30 14. Records of Erectile Response of Drooping Leaf and Stem after 15. Determination of Rate of Flow of Sap in Up and in Reverse

17. Record of Effect of Plasmolytic Solution in Arrest of Ascent of 21. ,, Arrest of Ascent of Sap at the Critical Temperature 66 25. Photographs of Normal and Poisoned Wheat-seedlings . . 73 tion of Poison . . . . • • 75 26a. Chrysanthemum. Photographs of Shoots in Water and in Formal 33. Curves Exhibiting Variation of Transpiration during Night 46. Diagrammatic Representation of Methods for Record of Exuda 47. Record of Diurnal Variation of Internal Pressure in Zea Mays

49. Record of Variation of Pressure in a leafless Tree {Poinciana 52. Record of Diurnal Variation of Pressure in Ciicurbita with Leaves 58. Record of Exudation and Pressure in the ' \^'eeping ' INIango 60. Phoenix sylvestris with Trunk sliced for Collection of Sap 62. Curve of Diurnal Variation of Exudation in Phoenix syliestri. 65. Electrometric Record for Determination of Velocity of Ascen 67. Method of Simultaneous Determination of Velocity by Mechan

69. Record of Electrical Pulsation of Desmodiitm . . JO. ,, Cellular Pulsations in Itnpatiens ... Cellular Pulsation in Naiiclea, by Einthoven-gal- vanometer . . . . . . .214 Amplitude of Electrical Pulsation at Different Layers . .217 Section of the Petiole of Brassica showing the Active Layer . 219 Method for the Determination of the Hydraulic Wave-length . 224 Record of the Determination of Wave-length in Chrysanthemiiin 226 Increase of Wave-length by Rise of Temperature 228 Effect of Passage of Electric Current on Variation

Cellular Contraction under External Stimulus . 235 Enhancement of Cellular Pulsation by Electric Effect of Electric Current on Cellular Pulsation . 237 Enhancement of Amplitude of Pulsation by Rise Effect of Chloroform on Cellular Pulsation . . 240 Arrest of Cellular Pulsation under Diminished Distribution of Fibro-vascular Tissue in the Stem of Papaya 247 Record of Dual Response in Averrhoa under Stimulus . . 250 Nerve-connections of the four Sub-petioles of Mimosa with the

Physical and Physiological theories — Inconclusive character of Stras- burger's poisoning and scalding experiments — Root-pressure. Among the fundamental activities in the life of the plant are the absorption of water from the soil and the conduc- tion of the sap to all parts of the body. By them the plant obtains its inorganic food-material from the dissolved constituents of the soil, and is supplied with the water necessary to maintain its cells in that state of turgor with- out which its growth and various life-movements would become arrested. Every portion of a tall tree has to be supplied with water, which is absorbed by the root, con- ducted along the stem, and finally excreted by the leaves. Calculations have been made which show that the amount of water transpired by the leaves of a large Birch-tree may be as much as 38 kg. per day. The energy required for lifting such large quantities of water to the top of the tree must be very great, especially when, as in the giant Eucalyptus amygdalina, it attains a height approaching 450 feet (150 metres).

The problem of the ascent of sap has, from the earliest days of plant-physiology, enlisted the keenest attention of numerous investigators ; but the results obtained have not yet been found to offer any wholly satisfactory solution of it. The obscurity of the subject is, in large measure, due to the presence of numerous co-operating agencies of but secondary importance ; the inquirer is very apt to be led into the error of confining his attention to one or other of these, thus missing the essential factor in the problem.

There is a voluminous literature on the different theories proposed in explanation of the ascent of sap, the enumeration of which here is out of the question : I must content myself with mentioning only some of the more important of them. They may be roughly classified as physical or physiological. According to the first, living cells take no part in the process. The second or physiological theory assumes, on the other hand, that the transport of water is fundamentally due to the activity of living cells, the movement being promoted secondarily by physical agencies.

Of the physical forces that have been invoked, obviously neither capillarity nor atmospheric pressure can offer any explanation of the phenomenon. There is a mainly physical theory, due to Dixon and Joly, and to Askenasy (1895), that has received more support than any other, according to which the ascent is brought about by the transpiration from the leaves. The fluid in the mesophyll-cells of the leaves becomes concentrated by evaporation. An osmotic attraction is thus set up in the leaves, and the suction thereby exerted is supposed to be transmitted back to the roots through cohering columns of water in the wood- vascular tissue. This theory labours under various difficul- ties. To begin with, it is inconceivable that slow osmotic action could produce a sufficiently rapid current of water. For I show, in the chapters on the subject, that the velocity of ascent may become more than 20 metres an hour, even in the complete absence of transpiration. There is, more- over, no conclusive proof that, under actual conditions, the water-columns within the plant could possess the necessary tensile strength : for the cavities of the wood- vessels and tracheides contain air-bubbles which must impair their cohesion. Ewart (1905) has shown that, in

Older to maintain the transport of water, a pressure- column five or seven times as great as the height of the tree would be necessary. He insists that the osmotic attraction developed in the parenchymatous cells of the leaf could not possibly exert so great a force. Turning next to the supposition that living cells may be instrumental in producing water-movement, Schwendener * assumes ' that the requisite energy is furnished in some as yet unexplained fashion by the living elements of the wood, thereby confessing his adherence to the views previously formulated by Westcrmaier, God- lewski, and Janse, who all maintain that the ascent of sap is a vital and not a purely physical process.' Godlewski postulated a periodic variation in osmotic pressure, during which the osmotically active substance is alternately broken down and built up afresh ; he was, however, ' un- able to prove this hypothesis. Hence no discussion of his theoretical conclusions is necessar}^ nor of those of Janse and Westermaier as to the way in which living cells may act in raising water. ' ^

Strasburger (1891-1893), on the other hand, en- deavoured to disprove the physiological theory by his experiments in poisoning and scalding trees. He showed that solutions of copper sulphate and of picric acid, in spite of their poisonous character, ascended to the top of the tree. He also killed portions of the stem by heat, and yet the uppei living and leafy portions were found to remain turgid for a few days. My experiments on the subject will be found in Chapter V ; they lead to a con- clusion diametrically opposite to that of Strasburger.

Strasburger's views have met with strong criticism from Pfeffer (1892) and from Ursprung (1904-5). Ursprung thinks that the living cells of the stem may, in some way, maintain the vessels in a favourable condition for conduction of water, or be instrumental in the ascent Physiological Plant Anatomy, Haberlandt, English translation, 1914, of sap. In support of this he carried out a series of experi- ments in which lengths of petioles and stems of plants were killed by the action of high or low temperatures, or by poisonous solu<-ions. He found that by killing por- tions of the stem, the wilting of the leaves above the dead area took place in the course of two to nineteen days, and that the greater the length of the stem that was killed, the earlier was the resulting wilting of the leaves.

It has been objected that the wilting of the leaves may not be due to the death of the intervening tissue, but to secondary reactions. Boehm believed that the wilting was brought about by the plugging of the vessels with mucilage. Dixon regards it as being caused by the intro- duction of poisonous or plasmolysing substances from the dead tissue. None of these various theories has been found to be completely satisfactory, as Pfeffer,^ in summarising them, points out :

' How and by what means the water is so rapidly trans- ferred even to the summits ot the tallest trees has not yet been satisfactorily explained, it has unfortunately not even been determined whether the aid of living cells is quite unnecessary.' ^ The experimental methods generally employed by observers labour under the disadvantage that long periods of time are required, which must necessarily introduce many complications. The wilting of the leaves, more- over, is a very crude index for the detection of induced physiological change. The ideal method would lie, not in the employment of average statistics, but in the quick measurement of the change in the rate of ascent of sap caused by some physiological variation. Such a method for the record of the ascent of sap would make it possible to subject the process to various crucial tests, which would decide once for all whether it is physical or