Doyle — Some Researches in Experimental Morphology. 431 



Sicher ersebeint mir nur, dass er an Bedeutung weit ziiriicksteht hinter dem 

 letzteu Faktor — den geanderten Stofileitungsvorgaiigen." He goes on tlien 

 to make a strong case in favour of the proposition that the transpiration 

 increase which gradually follows the increasing size of tlie sprout is responsible 

 to a mucli greater extent tliaii any other factor for the secondary develop- 

 ments in the petiole. Reasonable as such seems, are we yet in a position to 

 decide so definitely as Winkler has done ? Ai-e we not liable to ignore the 

 mechanical factor too much, and be too sure of a direct effect of transpiration 

 on vessel development, tending to regard this vascular development as a much 

 more simple process than it really is ? A short discussion of the points at 

 issue will, perhaps, indicate why it seems undesirable to take up so definite a 

 position. 



Thus the question of tlie effect of mechanical strains on the development 

 of the mecliauical tissue of a plant has produced some interesting results. 

 Hegler, quoted by Pfeffer (25, p. 127), says that petioles of Hellebonis 

 niger, with an original breaking strain of 400 grammes, were able to bear a 

 breaking strain of 3"5 kilogrammes, after they had been subjected to 

 increasing loads for five days. He describes, too, an increase in the 

 mechanical tissue. These results, however, have not been subsequently con- 

 firmed. Again, Ball (<51) was unable to find any response either by 

 meclianical tissue increase, or by increase in tlie breaking strain in a number 

 of plants grown with a continual tension acting on them. Wiedersheim (32) 

 subjected weeping forms of certain trees to continued tension, with no 

 response save in one case CovijUuh aveUana v. pendida, where he describes an 

 increase in the bast-fibre masses. Hibbard (33), too, found that tension 

 had no effect on the formation of mechanical tissue in stems, save in Vinca 

 major, where tliere was an increase in the amount of xylem, and in the 

 thickness of its cell-walls. Vochting (34), however, by growing a gourd in 

 such a way as to make the fruit-stalk support the weight, describes con- 

 siderable mechanical increase both in bast fibres and in wood-cells. This he 

 describes as due to a mechanical stimulus. " In diesem Beispiele zeigt sich 

 also der Einfluss der Belastung sobald sie als Eigengewicht in die Verkettuug 

 der korrelativen Vorgange eingreift. Hatte der liier gewonnene Soliluss 

 weitere Geltung, danu liesse sich das Entstehen der mechanischeu Grewebe 

 in den KnoUen der OxaJis [already often referred to] auf den Druck 

 zuriickfiihren, den die sich eutvvickelnden soheitelstandigen Triebe auf die 

 sie tragenden unteren ausiiben, einen Druck, der nun korrelativ vvirkte, d.h. 

 ganzlich verschieden von dem, welchen er als fremde, den objeotiven 

 angehangte oder aufgesetzte Last verursacht. Das Eigengewicht ausserte 

 einen functionellen Reiz in dem angegebeueu besoudereu iSiuue auf die Organe 



