126 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



fourteen years would be necessary 1 . Hence the accumulation and passive 

 secretion of methyl-blue would be extremely slow if the external fluid 

 were completely at rest. As a matter of fact, however, the currents set up 

 by changes of temperature, or by mechanical vibrations, are under normal 

 conditions sufficient to provide a continual supply of fluid without any great 

 expenditure of energy on the part of the plant being necessary. Both in 

 air and water (including the water of the soil), the operation of a variety of 

 natural causes ensures a continual and complete admixture of the whole, 

 and continually adjusts differences of composition between adjacent tracts 

 of air or water. Without this continuous readjustment, green leaves would 

 be unable to withdraw carbonic acid from the surrounding ocean of air with 

 sufficient rapidity, nor would the roots obtain a sufficiency of phosphates or 

 nitrates from the pure spring water in which they may be submerged. 



Differences of temperature, together with automatic or induced 

 plasmatic movements, suffice to procure continuous admixture within the 

 cell, and the water currents induced by transpiration are of the utmost im- 

 portance in plants which grow above ground. These water currents convey 

 water and dissolved substances over long distances, and even to the summit 

 of a tree, mainly through dead tissue-elements (Chap. X). Currents 

 passing in a particular direction hasten the transference of substances from 

 cell to cell, but nevertheless cannot compel any non-diosmosing material to 

 penetrate the 'protoplasm. 



In the above sense all movements are of importance. The streaming 

 of plasma is a directly visible movement, by which conspicuous admixture 

 and transference is possible within the cell. Indeed in some cases it may 

 be essential for the maintenance of sufficiently rapid exchange, although in 

 general plasma-streaming is not absolutely essential, for in many plants 

 it is not perceptibly active, even along the translocatory channels 2 . Other 



1 Stephan, Sitzungsb. d. \Vien. Akad., 1879, Bd. LXXIX, Abth. 2, p. 214. See also Ostwald, 

 Lehrb. d. allgem. Chemie, 1891, and ed., Vol. I, p. 697, and Winkelmann, Handb. d. Physik, 1891, 

 Bd. i, p. 604. On connected physiological questions, see Pfeffer, Energetik, 1892, p. 268. Since 

 diffusion is as rapid in soft gelatine as in water, for purposes of demonstration glass cylinders may 

 be partly filled with gelatine, and a coloured solution, such as indigo, poured over it, the diffusion of 

 the latter being directly visible and readily observed. If phenol phtalein and sodium carbonate (or 

 hydrochloric acid) be added to the gelatine, the diffusion of hydrochloric acid (or sodium carbonate) 

 may be followed (Ostwald, 1. c., p. 68). Agar and gelatinous citric acid may be used for the same 

 purpose. On the slow diffusion of certain aniline dyes, see Pfeffer, I.e., 1886, Bd. II, p. 302. 



The general importance of all mechanical admixing movement was first brought into promi- 

 nence by Pfeffer (Unters. a. d. Bot. Inst. z. Tubingen, 1886, Bd. II, p. 314, and Energetik, p. 270'. 

 De Vries regarded plasma-streaming as a necessary factor in translocation (Bot. Zeitung, 1885, p I). 

 In many cases, however, no marked streaming movements are normally perceptible, though as the 

 result of injury, &c., streaming may commence (Hauptfleisch, Jahrb. f. wiss. Bot., 1892, Bd. XXIV, 

 p. 173). On the absence of streaming in the sieve tubes, see Strasburger, Bau u. Verrichtung d. 

 Leitungsbahnen, 1891, p. 363. Protoplasmic connexions have been dealt with in Sect. 20. See 

 also Czapek, Sitzungsb. d. Wien. Akad., 1897, Abth. i, Bd. CVI, p. 155. 



