129 



In the first place, mechanical tisf5ues as well as fibro-. 

 vasoulay ones deserve our attention, because they seem better 

 fitted than otheo* kindB of tissue for experimental action 

 through increased demand upon them. Hegler was the first to 

 attempt to cause the formation of mechanical tissue by incteased 

 mechanical demand'^. The result of his investigations are the 

 following: the working power of plant organs is favored by me- 

 chanical strain since the mechanical elements are then more 

 abundantly formed than under normal conditions. Further, ac- 

 cording to Hegler, the production of meclmnical elements can be 

 excited by mechanical strain even in those organs which normally 

 develop none, X have mentioned already^ that these statements 

 of Hegler, at least on the plant organs which he Investigated - 

 petioles of Helleborus ni^er - are founded upon an errof , since 

 these even in aormal indivi'duals are not absolutely fred from 

 mechanical elements, as he thought them to be. 



In order to be able to furnish e clear picture of the al». 

 teration and strengthening of the mechanical tissues, I have 

 tried especially at different seasons of the year to influence 

 tissue formation in Helianthus seedlings by constantly effective 

 mechanical strain, * but unfortunately always in vain, so that 

 I cannot report upon the amount of strengthening which the me- 

 chanical elements undergo according to Hegler. A thorough test- 

 ing - the supplementing and correction - of his statements would 

 be most desirable, and would undoubtedly lead to interesting 

 conclusions, I wish, at this opportunity, to refer to the fact 

 that increase in the tr^chae capacity of any plcait part whatever 

 can be produced in other ways than by increase of tie mechanical 

 elements alone or. a better formation of the sepai'ate ones. As 

 is well knowh, mechanical demand upon the strained plant body 

 can change essentially the conditions of cohesion within it^, 

 (142) It does not seem impossible, that mechanical demand can also mod- 

 ify the specific conditions of cohesion, in the cellulose wall, 

 in the sense of an increase in mechanical effectiveness. 



Wiedersheim^ has recently investigated this thoroughly. He 

 let a heavy weight (as much ag 1.2kg^) act for months on branches 

 of different species of weeping t-rees and compared the formation 

 of the mechanical tissue in the strained and unstrained branche-s. 

 In most of the objects - the weeping varieties of Fagns silvatica . 

 Sorbus auouparia and F raxinus excelsior - no strengthening of 

 the hard bast was demonstrable; only in Corylus avellana could 

 an increase of the stereids be proved in the strained branches''. 



1 Pfeffer, Untersuch. R. Wegler's nb. d. Einfl. v. 2ug- 

 kraften auf . die Festigkeit und -die j?usbildung mechan* Gew, in 

 Pfl. Ber/ SSchs. Ges. WiS8,,1891. p, 638. 



^ Beitr. s?. Anat, d. Gallon Flora, 1900, Bd.LXXXVIl, p. 117. 



2 Compare for instance, Yillari. Ueb. d. Elasticit&t d. Kaut- 

 schuks, Poggendorf's Annalen d, Phys, u. Chemie, 1871,Bd.CXLIII. 



^ Ueb, d. Einfl, d. Belastung auf die Ausbildung v. Holz- 

 und Bastkorper bei Trauerbaumen . Pringsheim's Jahrb. f. wiss, 

 Bot., 1902, Bd. XXXVIII, p, 41. 



^ The stone cells do not participate in the strengthening tff 

 the mechanical tissues* "We found stone cells formed in the same 

 way in the laden as in unladen branches of Fraxinus, Fagus, Cor- 

 ylus (Widerscheim)". I' cannot find in the negative results of 

 the attempt to incite an increase of stone cells, any proof that, 

 as Wiedersheim assumes, these cells possess no mechanical signi- 

 ficance at least over against tensile strength. 



