TRANSLOCATION FROM CELL TO CELL 115 



such, as well as to any passive diffusion through their substance, the 

 relatively trifling total sectional areas of the threads would interpose a 

 marked hindrance 1 . 



Certain minor disadvantages are often the inevitable accompaniment of 

 any special modification for the performance of a particular function. It 

 might in some cases be essential that particular substances should be directly 

 transferred from the plasma of one protoplast to that of another, and for 

 this, the existence of plasmatic connexions would be of great value, although 

 their minute diameter necessitates the expenditure of a large amount of 

 energy to secure such transference. Thus in rhizoids, algal filaments, and 

 indeed in all cases in which the outer walls of a chain of cells are readily 

 permeable, there is an ever-present danger that substances which can 

 diosmose through the cell-wall may pass out and be lost in the surrounding 

 water or damp soil. The same risk is involved even when the protoplasts 

 only allow substances to pass through that portion of the cell-wall where 

 the two cells come into contact. In these and in similar cases we require to 

 determine how, and by what means, translocation is carried on without 

 loss. No such danger usually exists in the interior of tissues, where, 

 indeed, substances appear to wander to a large extent externally to the 

 protoplasts, but even here a restriction to special paths is possible, as will 

 be shown later (Sect. 108). 



Our knowledge of the mechanism of translocation, and especially of the 

 importance of the plasmatic connexions, is still very incomplete. With regard to 

 the latter, no decisive experiments have as yet been performed, although according 

 to certain authors 2 , translocation takes place, mainly, or entirely, through the 

 connecting plasmatic threads. Such views, however, cannot claim to be based 

 upon a careful consideration of all the facts observed. Since the thinnest threads 

 will suffice to maintain living continuity, the coarse thick connexions observed in 

 the case of the sieve tubes may indicate that the plasmatic strands are here mainly 

 employed in the transference of substances from one segment of the sieve tube to 

 another. 



With morphological relationships we are not concerned, nor is it necessary to 

 discuss whether the threads are of secondary origin or are present from the 

 commencement, persisting during cell-division between the nodules of cellulose 

 which unite to form the cell-plate or dividing wall 3 . That a secondary formation 

 of connecting threads may be possible is indicated by the fact that in certain 

 cases the living protoplasm has been shown to be capable of boring its way 

 through cell- walls. 



1 Pfeffer, Studien z. Energetik, 1892, p. 272. 



a Thus Kienitz-Gerloff (see Zimmermann, I.e., p. 331". Kienitz-Gerloff gives an account of 

 the views held with regard to the transmission of stimuli by the plasmatic threads, 1. c., 1893, p. 49. 

 Cf. Pfeffer, Ber. d. Sachs. Ges. d. \Yiss., 1896, p. 505. 



3 Zimmermann, 1. c., p. 330. 



I 2 



