036 



STIMULUS-TRANSMITTING SYSTEM 



B 



At the present time, the majority of investigators incline to the 

 belief that the closing-membranes of all the pits developed between 

 living cells of the plant-body are perforated by protoplasmic connections, 

 while many hold the opinion that all the living cells of a plant are 

 linked together by means of connecting threads, so as to form a single 

 protoplasmic entity or " symplast " ; but Kuhla's work on Viscum album 

 represents the only attempt that has been made so far, to prove the 



truth of this assumption, in the case of 

 a highly organised plant. Attention may 

 therefore be drawn to the fact that, in all 

 the cases hitherto examined, organs engaged 

 in the perception of external stimuli are 

 connected by means of protoplasmic threads 

 with the associated reactive region or motor- 

 tissue. Thus, Pfeffer 334 has demonstrated 

 the presence of these structures in the par- 

 enchymatous tissues of tendrils, in which 

 transmission of stimuli takes place, though 

 not indeed over any great distance ; con- 

 necting threads also occur in the walls 

 separating the epidermal sensory cells of 

 tendrils from the hypodermal parenchyma. 

 The author has observed that the thickened 

 inner walls of the sensory cells in the 

 staminal filaments of Berberis vulgaris 33 * 

 are furnished with fairly numerous shallow 

 often very ill-defined pits, the compara- 

 tively thick limiting membranes of which 

 are traversed by protoplasmic threads (Fig. 

 236 a). Each sensory cell is thus able 

 to transmit the excitation set up by the initial stimulus to the 

 adjacent motor-tissue, the walls of which are also liberally provided 

 with pits and protoplasmic connections. In the case of Aldrovandia 

 vesiculosa 3 ^ the lower transverse walls of the sensory cells in the tactile 

 hairs are devoid of pits ; they are, however, relatively thin as regards the 

 portion which lies next the central axis of the hair. By suitable treat- 

 ment, the author has succeeded in demonstrating the presence of a small 

 number of protoplasmic threads in the thin portions of these walls ; as a 

 rule, there are but two or three such threads in each wall, or even 

 only a single one. Where the " hinge " consists of four cells, therefore 

 communication with the neighbouring protoplasts is maintained with 

 the aid of from four to twelve connecting threads. In the case of 

 Dionaca muscipula, protoplasmic connecting threads have been found by 





Fig. 265. 



A. Protoplasmic connections be- 

 tween two cells of the mesophyll of 

 Piscum album (after Kienitz-Gerloff). 

 B. Curved protoplasmic connections 

 in one of the radial walls of the 

 aleuroue layer of Zea Mais, after 

 treatment with iodine and dilute 

 sulphuric acid. 



