434 Scientific Proceedings, Royal Dublin Society. 



This would reduce the stem-wood to about 8 sq. muis. or only one-third the 

 quantity of wood developed in tlie petiole. And a possible suggestion to me at 

 any rate is that the petiole running out at right angles to the stem, and carry- 

 ing a very large plant upon it, is subject to most abnormal conditions of 

 mechanical strain. The large wood-development is the result of this stimulus. 

 Now if we glance back at Plato XXVTII, fig. 5, we see a swelling at the base 

 of the petiole running into the si em. The wood here was also measured 

 —although very roughly — and was fouud to be in area more tlian twice 

 that in the petiole. Tiiis is surely a large development, hardly to be 

 accounted for by any claims of water conduction. The closer anatomy of the 

 secondary wood in all tlie petioles, as well as the abnormal wood in the 

 Solanums, must be carefully investigated as soon as possible. 



As a result of this survey it must be admitted that tliei-e is sufficient 

 evidence in favour of the proposition that mechanical stimuli are a very 

 important factor indeed in the development of the secondary wood in our 

 petiole, to indicate the necessity of further and immediate investigations to 

 confirm the point. 



Even if we grant the mechanical factor is not only more important than 

 admitted by Winkler, but may even be of considerable importance absolutely, 

 yet there is still some other powerful factor at work. Indeed, its effect may 

 well be much greater than that of the mechanical factor, but the relative 

 importance of the two can hardly be decided in our present state of 

 knowledge. However, this other factor is certainly intimately connected 

 with the large foliage development. Winkler decides very definitely that 

 the xylem increase is directly dependent on the gradually increasing trans- 

 piration of the growing shoot. He denies tiiat the transpiration effect is 

 brought about through any changes in the water-content of the plant, but that 

 it is a functional stimulus acting indirectly on the cambium. The stimulus 

 is carried to tlie cambium by the living tissue of medullary rays, and 

 apparently originates in the vessels — that is, if a vessel, owing to increasing 

 transpiration, has to carry a larger quantity of water tlian usual, then a 

 stimulus passes to cambium to form more vessels. In his own words, the 

 actual factor is " der Grad der Inanspruchnahme der Gefasse." Now this 

 is an interesting theory, but it would seem to have been built on too 

 few facts. It is, indeed, quite possible that it expresses with some accuracy 

 the state of things, but surely our accurate knowledge of the actual causes of 

 secondary xylem development is too incomplete to really warrant the building- 

 up of any cut-and-dry theory. 



In the first place, one can readily admit that tliere is some connexion 

 between leaf-development and that of secondary xylem. The work by Jost 



