46 

 plaiits with sodium chlorid.-'' 



(4Q) Oonducting and Mechanical Tissues. Even in the dis- 



^ cussion of the axis much may be said concerning the differ- 

 entiation of the epidermis , of the assimilatory tissue in 

 the bark, etc. In general, all that was said in the dis- 

 cussion of leaves also holds good here. In the same way 

 a consideration of the conducting and mechanical tissues, 

 which in general determine the histology of the axis, fur- 

 nishes nothing essentially nev/. In arrested development 

 of the axes, the vascular bundles decrease in number, the 

 individual bundles are impoverished, the equipment with 

 mechanical protecting sheaths degenerates or disappears 

 entirely; instead of connected "mechanical rings", isolated 

 groups of thick-vi^alled elements are produced, and the col- 

 lenchyma cords in the bark occur sparingly or not formed at 

 all. The same reduction of tissue-differentiation may be 

 obtained in roots, as in axes. In these the same factors 

 are everywhere decisive as in the reduction of mesophyll 

 differentiation, etc. The investigations of Thouvenin 

 deserve*^ especial mention; he retarded the development 

 of the tissues by the action 6i mechanical pressure. The 

 mechanical tissues in the stem of the Zinnia remained be- 

 lov; the standard, 



Zalenski^ showed, further, that the length of the 

 vascular bundles, calculated by the surface of the leaf- 

 blade, is dependent upon external factors in such a way, 



1. Compare also the cultural experiments of Brick, 

 Beitr. z, Biol. u. vergleich. Anat. d. baltischen Strandpfl. 

 Schr. Naturforsch. Ges. Danzig. 1888, N.F.Bd. VII, 1, Heft, Glaus , 



2. The above quoted authors. Bonnier, Borgesen, Costan- 

 tin, Dufour, Gauchery, Hartmann, Keller, Kohl''; lothelier, 

 Rauwerihoff, Schenck, Stapf , Teodoresco, Yes que and Yiet 

 ' should be compared. Compare further Perseke, Uber die Form- 

 veranderung d. V/urzel in Erde u. Wasser, Dissertation, Lei- 

 pzig, 1877. Costantin. Infl, du sejour sotis le sol sll. 

 struct, anat. d. tiges. Bull. Soc. Bot., France 1883, T. JUUi, 

 p, 230. Et. comp. d. tiges. aerienneo et sout. d. Bicotyl, 

 Ann. Sc. Hat, Bot., 1883, VI, serie, T. XVI, p.. 4. Rech. s.l. 

 struct, de la tige d. pi. aquatiques. Ibid., 1884, VI, serie, 

 T. XIX, p. S87, Dassonville, Action des sel^' ^' 1%*°™® 

 et la structure d. veget. Rev. gen. de Bot., 1896, i. viii, 

 p. 284, and 1898-, T. X., p. 15. Farmer and Chandler espec- 

 ially (on the influence ox carbon dioxide etc. Proc. H. boc. 

 1902, Vol. IXX, p. 413) made investigations on the miluence 

 exerted by a superabundance of carbonic acid . 



3, Des modifications apportees par une traction long- 

 itudinale de la tige, C.R. Acad. Sc. Paris ,1900, T.CXXX, p. 663. 



4. Ueb. d. Ausbildung d. Nervation b. verscheid. Pflan- 

 zen. Ber. d, D. Bot. Ges, 190E, Bd. XX, p. 433. 



