640 STIMULUS-TRANSMITTING SYSTEM 



threads. The conditions are, of course, even more favourable for rapid 

 transmission where the transverse septa are oblique, or in other words, 

 where the transmitting elements are prosenchymatous. Stimuli might 

 consequently be expected to travel more rapidly along vascular bundles 

 than through parenchymatous tissues. As a matter of fact, Frank 

 found that, in Modea canadensis, the rearrangement of chloroplasts 

 induced by a transmitted traumatic stimulus begins sooner in the 

 vicinity of the midrib, than it does in other parts of the leaf. 

 Kfetschmar likewise states that transmission takes place most rapidly 

 in the vascular bundles, in a number of water-plants. In a leaf of 

 Vallisneria which was severed at a distance of 4 cm. from the anex, the 

 traumatic stimulus that induces protoplasmic streaming was found by 

 Kretschmar to travel to the apex in ten minutes along the midrib, 

 and in twelve minutes along the two lateral veins ; twenty-six minutes 

 elapsed before all the mesophyll cells were engaged in streaming, and 

 it was evident that the stimulus reached the mesophyll principally 

 through the veins. Nemec states that the traumatic stimulus which 

 leads to a rearrangement of the protoplasm in the root-tip of Allium 

 Gepa (cf. above, p. 23), is propagated most rapidly in the inner layers of 

 the periblem and in the plerome. It is therefore quite to be expected 

 that vascular tissues should be employed for the conduction of stimuli, 

 where ecological considerations demand a high velocity of transmission. 

 As a matter of fact, Batalin 340 states that stimuli applied to the tactile 

 bristles of Dionaea muscipula travel towards the midrib mainly along 

 the vascular bundles. In the case of Biophytum scnsilivum, 341 the 

 author's own observations render it probable that both mechanical and 

 traumatic stimuli are likewise propagated along the vascular strands of 

 the pinnae and rhachis. Transmission of stimuli in vascular bundles, 

 in so far as it is effected by protoplasts and protoplasmic connections, 

 must take place in the parenchymatous elements of the hadrome- and 

 leptome-, and perhaps also in the living contents of the sieve-tubes, 

 while in the case of leaves, the cells of the parenchymatous bundle- 

 sheaths may also be utilised for this purpose. 



The greater velocity of transmission in vascular bundles is perhaps 

 not altogether due to the prevailing elongation of the component cells ; 

 it is possible that the protoplasmic paths of transmission are here 

 endowed with a special molecular or micellar structure which renders 

 them more efficient in this respect than those of the parenchymatous 

 ground-tissue. 



Where there is no special need for rapid propagation of stimuli, the 

 transmitting capacity of the general parenchyma may be quite 

 sufficient. In the cotyledonary sheath of Avena saliva, for example, 

 heliotropic stimuli are propagated, according to Eothert, 342 with sufficient 



