430 Scientific Proceedings^ Royal Dublin Society. 



fig. 3. Here we meet enoinious and fundamental clianges in the pliysio- 

 logical conditions of the petiole, and here consequently the main causes 

 of the secondary thickening must be sought. We take the increase in 

 conduction of plastic substances first. Thus De Vries (17) claims that the 

 secondary developments in tlie fuber of Oxalis, caused to function as a stem, 

 are due to a current of such nutritive material — the cause is to be sought " in 

 der Eewegung der NahrstofEe im Xylem und in den Siebrohren, sowie dem 

 benachbarten Parenchym." He particularizes this Nahrstoff later as 

 Eiweissstoffen and Kohlehydraten. Now, apart from tlie fact that it seems 

 hard to see wliy cambium cells should be stimulated to sucii activity by a 

 " Bewegung " of these materials wlien a more passive storage, altliougli an 

 equal nutritive excess, sliould merely give rise to storage parenchyma, we 

 find De Vries vigorously combated by Jost (27) and (28). He demonstrates 

 that in seedlings of Phaseolus, as already mentioned above, with plumule 

 removed, there is a continual stream of such material passing from the 

 cotyledons to tlie epicotyl, and tlienoe after a short time to the developing 

 roots while vascular development is all but absent. He adduces, too, such 

 facts as tliat secondary thickening in trees stops before autumnal leaf-fall, 

 and thus while a nutritive stream is still passing in the neighbourhood of the 

 cambium, concluding that the nutritive stream "ist nur eine Bedingung 

 niemals aber die TJrsache des Wachstums." Lately Snell (30) has re-opened 

 the question of the effect of the nutritive streaming on vascular development, 

 but he has not made any criticisms of Jost's objections to the nutritive 

 current theory, still less does he adduce any further experimental evidence 

 in its favour. So that the movement of large masses of food material in the 

 neighbourhood of a cambium would seem to have no influence on the develop- 

 ment of secondary tissue. From a priori reasons it does not seem improbable 

 that changes of turgor in the cambium cells should result from such a 

 nutritive stream, and that cell-wall thickenings might in some way result 

 therefrom; but pending further general research along these lines, we certainly 

 cannot claim a positive influence for this factor. An active upward move- 

 ment of such material, apart from the parenchyma of the bundles, would be 

 so closely bound up with the transpiration stream which will presently be 

 discussed as not to need discussion here. 



The Stimuli of increased Mtclianical Strain, and of increased Water Conduction. 



These two factors can be more conveniently treated together. 



Winkler (5) says, p. 57, speaking of mechanical strain increases — " Ich 

 mochte es als wahrseheinlich bezeichnen dass der mechanisohe Faktor dabei 

 eine Rolle spielt, wenn sich diese vorerst auoh nicht naher prazisieren lasst. 



