THE MECHANISM AND CAUSES OF TRANSLOCATION 589 



sieve-tubes seem, however, to be used to a marked extent for direct trans- 

 location from one sieve-tube segment to another, and a very slight pressure 

 suffices to cause currents to flow in a given direction through a sieve-tube 

 (cf. Sect. 20). 



Since diffusion takes place with great slowness, currents in mass and 

 mechanical admixture are of the utmost importance in ensuring rapid 

 transference, for if a substance becomes rapidly distributed throughout 

 a cell immediately after its entry, the transit to the next cell requires 

 only a comparatively short time, no matter whether the passage from 

 one cell to the other is effected diosmotically or by means of protoplasmic 

 connexions. Hence granted similar osmotic powers, longitudinal trans- 

 ference will be much more rapid through prosenchyma than through 

 parenchyma, where the number of partition-walls to be passed is much 

 greater. It is perhaps mainly for this reason that translation is most 

 rapid in the phloem, and the coarse perforations of the sieve-plates render 

 the sieve-tubes especially adapted for this purpose (Sect. 106). 



A sufficiently rapid admixture is attained within the cell by mechanical 

 swaying or bending movements of the entire plant, by changes of 

 temperature, by variations of turgidity or of tissue-tensions, &c., while 

 streaming movements of the plasma are especially important, although 

 these do not occur in many cases under normal conditions, and are usually 

 not exhibited by the sieve-tubes or conducting tissue of the phloem. 

 Hence sufficiently active translocation is possible without the aid of any 

 marked streaming movements in the plasma, which de Vries l erroneously 

 regarded as of universal and indeed of decisive importance in this respect. 

 Whenever streaming movements are of normal occurrence, they are 

 primarily of importance in securing the rapid distribution of substances 

 within the cell, for apparently the plasma of the connecting threads is 

 stationary, or at any rate not in such active movement as would be 

 necessary to convey large quantities of substance from cell to cell 2 . Even 

 marked unilateral pressure is insufficient to induce any streaming movement 

 in mass through the minute canaliculi occupied by the connecting plasmatic 

 threads, whereas this readily happens through the coarse pores of the 

 sieve-plate, as is shown by the fact that when the sieve-tubes are opened 



1 De Vries, Bot. Zeitung, 1885, p. i. Cf. Pfeffer, Landw. Jahrb., 1876, Bd. v, p. in ; Ener- 

 getik, 1891, p. 269, and Sects. 20 and 22. De Vries supposed that plasma-streaming was of general 

 normal occurrence, from observations made on sections, whereas Hauptfleisch has shown (Jahrb. f. 

 wiss. Bot., 1892, Bd. xxiv, p. 173) that streaming frequently commences in response to the stimulating 

 effect of injury, or is brought into prominence by the action of special external stimuli. On the 

 absence of plasma-streaming in sieve-tubes, cf. Strasburger, Leitungsbahnen, 1891, p. 363. 



2 Cf. Pfeffer, Energetik, 1891, p. 272. Czapek, Ber. d. Bot. Ges., 1897, p. 128 (Sitzungsb. d. 

 Wien. Akad., 1897, Bd. CVI, Abth. i, p. 155). Kienitz-Gerloff's discussions (Bot. Zeitung, 1893, 

 p. 36) do not alter the matter as here represented. 



