Structural Botany : The Transpiration Current. XI. 



Absorption of Water by the root-hair is regarded as mainly a matter of 

 osmotic activity, dependent on the solutes of the cell-sap and the control of the 

 semipermeable plasmatic film ; the osmotic pressure of the cell being normally much 

 greater than that of the external dilute solution. Capacity for absorption implies 

 a similar capacity for getting rid of excess fluid ; and this when passed to subjacent 

 parenchyma implies Conduction. The physical mechanism of plasmatic conduction 

 is wholly obscure ; speculations in terms of osmotic mechanism remain unsatisfactory, 

 as implying that each cell is an isolated mechanism. Colloidal plasma is undoubtedly 

 freely permeable for water as the original medium of its existence, and continuity of 

 the medium through colloidal membranes implies molecular continuity of the water 

 all the way. The fact of absorption by the root-hair implies turgidity oFthe entire 

 range of living cells; i.e. living units freely conduct until all are turgid; cells losing 

 water draw freely on the others ; younger units drain older ones ; as in the familiar 

 process of putting flagging cut-flowers in water, by stalks or tops. 



A new departure takes place at the first dead tracheide : here a mechanism of 

 discharge or active secretion must be postulated. Plasma of land-plants necessarily 

 secretes water as a necessity of existence ; there being no other way of obtaining 

 food-salts in quantity from a dilute solution (cf. Myxomycete plasmodia secrete drops 

 of fluid, Dry Rot Fungus, Merulius lachrymans, Water-glands of leaves, and water- 

 pitchers of Nepenthes). In connexion with such secretion of water into a dead 

 * conducting' tracheide, it may be noted (i) the tracheide was full to begin with, and 

 (2) there is no evidence that it was dead or empty when it began to conduct. Such 

 secretion into the tracheidal system marks the beginning of the 



Transpiration Current, and further rise of water in the dead xylem is mainly 

 physical ; the system is full to begin with, and remains filled ; fluid being drawn off 

 above by the transpiring cells, much as the root-hairs below draw on the soil-solution ; 

 as loss is made good, water moves in the stem in the dead tracheides. Such a tracheidal 

 system, more perfectly expressed in terms of vessels, is in full communication throughout 

 the plant or even a tall tree. In a tree 300 ft. high, the water-column with a pressure 

 of 10 atmospheres would only just neutralize the osmotic pressure of the turgid cells 

 at the base of the system with 10 atmospheres capacity. The height to which water 

 can rise is thus probably connected with the osmotic possibilities of the parenchymatous 

 conducting cells ; while owing to the wet colloidal walls there is practically no waste of 

 energy in friction. Lignified walls cannot be imitated by glass tubes. 



Root Pressure : A special case of intensely active absorption, conduction, and 

 secretion is seen in the rise of fluid in the tracheal system of many trees, more 

 particularly in the spring ; this being noticed before the new leaves appear as a ' rise 

 of sap ' (cf. Sugar-Maple, Birch), and is measured as a positive pressure by water- 

 column, mercury manometer, or pressure gauge (e.g. Vine 1,000 mm. Hg; Birch 

 2^ atmospheres): when the leaves are expanded, a negative pressure is generally 

 noticed. Such activity is not confined to the root ; and ' bleeding ' may be induced 

 in other parts with active parenchyma; cf. Arenga (Toddy Palm) * bleeding* induced 

 by blows, long-continued, and the fluid contains sugars. 



Absorption of Salts is independent of osmosis, the latter being solely a pheno- 

 menon of water-pressure. In a dilute solution the inorganic solutes are practically 

 completely ionized ; diffusion is assisted by the colloidal plasma adsorbing ions and 

 complex molecules with little selective capacity ; e. g. poisonous substances are taken 

 freely, as ions of CuSo 4 , and dyes as Methylene Blue. 



Selective Absorption follows in the long run, as only the substances removed 

 from the solution in metabolism create a drain lor more of the same kind, to give an 

 absorption equilibrium. Hence from the same solution different plants give a net 

 absorption differently for the same salts. There is no excretion of osmotic material 

 from an undamaged root-hair ; but an acid effect (as seen by corrosion of marble 

 slabs) may be the effect of (i) contents of dead hairs, (2) excretion of CO 2 in respira- 

 tion, (3) excessive removal of basic ions (Ca", Mg", K', Na') in metabolism. 



Culture Solutions : by the omission of one or more constituents at a time, 

 it is possible to trace the possible functions of different materials : e. g., omission of 

 Iron renders green plants chlorotic, owing to failure of chlorophyll-production, although 

 chlorophyll contains no iron. Lack of Potassium stops the photosynthetic mechanism, 



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