140 THE CAUSES OF SPECIFIC SHAPE 



internal disposition and the factors which regulate it. In certain cases, 

 however, the root-apex may be transformed into a shoot-apex, and vice 

 versa, a fact which affords further evidence of the inherent similarity between 

 the meristems of the root and shoot. 



This transformation occurs normally in the root of Neottia Nidus-avis, whose 

 apex, after forming leaf-primordia, throws off the root-cap and grows onwards as 

 a stem 1 . The same has sometimes been observed in the root-apex of Anthnrinm 

 longifolium 2 , and in cuttings of Selaginella the lower primordia of rhizophores 

 frequently develop into roots, the upper ones into shoots 3 . Beyerinck 4 has ob- 

 served on a variety of plants, more especially on Rumex Acetosella, the conversion 

 of root-primordia into shoots, and the transformation of an embryonic adventitious 

 bud bearing rudimentary leaves into a root. Such changes are aided by removing 

 so much of the plant that the stem- or root-primordia are left in positions where 

 the opposite kind of organ would normally be developed. 



A good instance of the influence of the preformed parts is shown by the 

 thallus of Marchantia, in which the dorsiventrality, when once induced, can- 

 not be reversed in the growths formed under altered conditions, although 

 such reversal is easily induced in the prothalli of ferns. 



The internal disposition is not necessarily the same in all parts of a given 

 plant, and hence it is not surprising that external conditions which cause 

 embryonic cells in the older parts of the stem to produce roots, may not be 

 able to induce the formation of roots on buds. In every case it can only 

 empirically be determined whether an induced alteration of disposition is 

 labile and changeable by varied external conditions, or whether the induction 

 is stable and permanent 5 . The latter is the case when internal or external 

 agencies induce the formation of the primordium of a root or shoot, which 

 passes on its own characters to the meristematic apex, and so determines 

 the entire subsequent development of the shoot or root. The same also 

 applies to the growing thallus of Marchantia, in which the dorsiventrality, 

 when once induced, is impressed upon all subsequent growths. 



Instances of labile induction are afforded when the changed shape or 

 structure induced by an orienting stimulus disappears in the new growths 

 formed after the stimulus has been removed. No sharp distinction can, 

 however, be drawn, for an instance of an apparently stable induction may 



1 Irmisch, Biol. d. Orchideen, 1853, p. 26 ; Prillieux, Ann. d. sci. nat., 1856, 4* se>., T. V, p. 279 ; 

 Beyerinck, Beobacht. u. Betracht. ii. Wurzelknospen n. Nebenwurzeln, 1886, p. 17. Beer states 

 that the same occurs in the orchid Catasctum tridentatum (quoted by Irmisch). 



3 Goebel, Bot. Ztg., 1878, p. 645. Karsten (Flora, 1861, p. 232) observed the development of 

 a flower from a root-apex. 



3 Pfeffer, in Hanstein's Bot. Abhandl., 1871, 1, Heft 4, p. 67 ; Beyerinck, 1. c., pp. 3, 16; Behrens, 

 Flora, 1897, Erg.-bd., p. 158. Behrens observed the same change on Selaginella denticiilata, which 

 possesses true roots. 



* Beyerinck, 1. c., pp. 13, 42 ; Philippi, Ber. d. Bot. Ges., 1901, p. 95. 



* Pfeffer, Pflanzenphysiologie, i. Aufl., Bd. n, p. 163. 



