176 THE CAUSE OF SPECIFIC SHAPE 



by grafting a shoot of a perennial plant upon it \ On the other hand, the time of 

 flowering of a beet-stem which has not yet flowered may be determined, and hence 

 its duration increased or shortened, according to whether it is transplanted upon 

 an old or young beet-root. 



It is owing to the dormant potential powers of the leaf-stalk of Vitis vinifera 

 of responding to increased demands that it grows considerably in thickness when 

 a shoot is grafted upon it 2 . The same takes place, according to Carriere, when an 

 orange-shoot is grafted on a leaf-stalk, and the duration of the latter is prolonged 3 . 

 Such flowering axes as those of Pelargonium^ which may become converted into 

 a permanent leafy axis capable of growth in thickness 4 , would probably behave in the 

 same manner when a leafy shoot was grafted on them. 



Knight, Tschoudy, and Gartner all showed that in general only closely related 

 plants can be successfully united by grafting. In many cases the shoots, even 

 of the same plant, only unite with difficulty, whereas the potato may have not only 

 various Solanaceae grafted upon it, but even a member of the Scrophulariaceae, 

 Schizanthus Grahami b . In the case of suitable plants organs of different mor- 

 phological value may be grafted together, and a piece of root may even be caused 

 to take on the conducting function of the portion of stem it replaces. Vochting 

 has shown that the union is either incomplete or fails entirely when the graft is 

 attached in an inverted position. This is due to the polarity of the pieces of stem 

 or root employed, for in the inverted position translocatory exchanges are less 

 readily effected. In bud grafting, however, the bundles differentiate in the callus- 

 tissue along a curved path, so that the interpolated piece of bark functions for 

 translocation in the normal direction. 



SECTION 50. Details concerning Symbiotic Unions and Interactions. 



Lichens, the root-nodules of Leguminosae, the yellow cells of 

 Radiolaria, are all instances of symbiotic association between widely 

 dissimilar organs, whereas in flowering plants grafting succeeds only 

 between closely related organisms. The parasites Viscum and Orobanche^ 

 however, afford instances of symbiotic union between Phanerogams be- 

 longing to widely dissimilar families. There is in fact no parallelism 

 between sexual and symbiotic affinity, a principle established by Gartner 

 long ago for flowering plants 6 . 



It can therefore only be determined by experiment whether two 



1 Vochting, Transplantation, 1892, p. 85. On the union of deciduous and evergreen plants cf. 

 Daniel, Compt. rend., 1897, Bd. cxxv, p. 66 1. 



2 Knight, Phil. Trans., 1804, I> P- l8 9- cf - Vochting, 1. c., 1892, p. 78. 



3 Quoted by de Vries, Jahrb. f. wiss. Bot, 1891, Bd. xxn, p. 49. 

 * De Vries, 1. c., p. 50. 



5 Noted by Tschoudy in 1819. Cf. also Vochting, 1. c., 1892, pp. 18, 23; Strasburger, Ber. d. 

 Bot. Ges., 1885, p. 34 ; Lindemuth, Gartenflora, 1897, p. 5 ; H. Molisch, Bot. Jahresb., 1897, p. 155. 



6 Gartner, Vers. u. Beobacht. ii. die Bastarderzeugung im Pflanzenreich, 1849, p. 629 ; Vochting, 

 Transplantation, 1892, p. 23. 



