PROTOPLASMIC CONNECTING THREADS 637 



Macfarlane and by Gardiner, both in the tactile bristles and in the 

 general niesophyll. Gardiner has also noted the occurrence of these 

 structures in the lower, or sensitive half of the pulvinus of Mimosa 

 pudica ; they seem, however, to be quite as plentiful in the upper, or 

 insensitive half of the pulvinus. According to Czapek and A. W. Hill, 33 

 root -tips, which are concerned in the transmission of geotropic, hydro- 

 tropic and perhaps also heliotropic stimuli, contain an unusually large 

 number of connecting threads ; moreover, the manner of their dis- 

 tribution in these organs distinctly supports the notion that the threads 

 are responsible for the conduction of stimuli. 



Direct experimental demonstration of the stimulus-transmitting 

 function of protoplasmic connecting threads is naturally a matter of 

 the greatest difficulty. Townsend, 338 working with Moss-protonema fila- 

 ments and with hairs, has shown that the nucleus of one cell can induce 

 an isolated non-nucleated mass of cytoplasm, contained in a neighbouring 

 cell, to surround itself with a cell-membrane, if the two protoplasmic 

 bodies are connected by protoplasmic filaments. While in this case it 

 is not an external stimulus that is propagated, it is reasonable to suppose 

 that the transmission of nuclear " impulses," and of internal stimuli in 

 general, does not differ in principle from the propagation of external 

 stimuli. Strasburger makes use of the fact that the majority of the 

 protoplasmic connections are retracted or broken when a cell is plas- 

 molysed, and fail to be regenerated when it is allowed to resume the 

 turgescent condition. A root or stem is first completely plasmolysed by 

 immersion in a suitable salt-solution ; it is then thoroughly washed, 

 allowed to become turgescent again, and finally tested with regard to 

 its power of executing geotropic curvatures. Organs treated in the 

 manner described are found to be no longer capable of geotropic response. 

 It is certainly jjossible that this result is partly due to the rupture of 

 the stimulus-transmitting protoplasmic connections. But it is just as 

 likely that plasmolysis destroys the perceptive capacity or upsets the 

 mechanism of curvature, or interferes with some other link in the chain 

 of stimulation. Strasburger's experiments cannot therefore be regarded 

 as conclusive, as he himself indeed admits. 



It may be presumed that the protoplasmic connecting threads do not 

 confine themselves to the propagation of external stimuli, or of the 

 excitations produced by the latter, but that they are also concerned in 

 the transmission of those internal stimuli which play so important a 

 part in the mutual relations of different tissues and organs. It has, in 

 fact, already been mentioned, that the influence of the nucleus upon the 

 formation of cell-walls can be transmitted from one cell to the other by 

 means of protoplasmic connections. 



Whether one and the same connecting thread is capable of conducting 



