PROTOPLASMIC CONNECTING THREADS 635 



cellular transmission, while the protoplasmic threads connecting adjoin- 

 ing protoplasts with one another represent the paths of intercellular 

 transmission. 



The differentiation of sensory and motor nerve-fibres which 

 characterises the paths of stimulus-transmission in the animal organism, 

 is correlated with the presence of a central nervous organ or reflex- 

 centre. This organ is absent or at any rate has so far remained 

 undiscovered in the vegetable kingdom. Hence, sensory and motor 

 paths of transmission are not distinguishable in plants, although there 

 must of necessity be a point somewhere in every path of transmission, 

 at which the state of excitation produced by the stimulus becomes 

 transformed into the impulse that initiates the responsive movement. 



A. INTERCELLULAR TRANSMISSION. 



The protoplasmic connecting threads discovered by Tangl 333 (termed 

 Plasmodesmen by Strasburger) generally take the form of extremely 

 delicate protoplasmic filaments, which traverse the whole thickness of 

 the cell-walls and thus place the protoplasts of adjoining cells in 

 direct communication with one another (cf. above, p. 45). As a rule 

 large numbers of these threads traverse the closing-membranes of pits 

 (Fig. 265a); in the cortical parenchyma of Viscum album, for example, 

 Kuhla often counted more than twenty connecting threads in each 

 closing-membrane. Less frequently, the more or less thickened 

 unpitted regions of the walls are perforated by solitary connecting 

 threads. The two modes of distribution of the threads are, however, 

 not sharply separable from one another ; for the same cell-wall may 

 be traversed both by groups of connecting threads and by isolated 

 protoplasmic filaments. 



It is highly probable that connecting threads consist of the same 

 hyaloplasm that composes the outer plasmatic membrane or ectoplast ; 

 they may in fact be regarded as prolongations of the ectoplast. 

 Evidence in favour of this view, which was first put forward by Noll, 

 has been accumulated by Strasburger ; this author points out that the 

 diameter of the connecting threads, which are notoriously very slender, 

 never exceeds the thickness of the ectoplast, so far at any rate as the 

 latter can be measured ; further, ectoplast and connecting threads are 

 not only directly continuous with one another, but also consist of the 

 same substance. 



As regards the origin of the protoplasmic connecting threads, 

 Strasburger has shown that they are not, as might be supposed, 

 persistent spindle-fibres, but secondary formations, interpolated after 

 the cell-wall has come into being, although they appear at a very early 

 stage, before the deposition of the secondary thickening layers. 



