390 ORGANIC GROWTH sec. 



those in wliicli obvious external stimuli directly excite the new 

 process of growth ; (2) those in which no such excitation occurs. 

 Among the first group is the growth into new plants of parts 

 of a plant when cut off and brought under favourable external 

 conditions — such as cuttings which send forth roots when 

 they are set in the earth, and grafts. Grafting can with 

 certain limitations be performed on the bodies of animals, 

 including man. I allude to the transference of pieces of 

 another person's skin to the human head, or to other parts of 

 the body-surface (transplantation).-^ 



A similar phenomenon, although it does not fall under the 

 definition of the term grafting, is the readhesion of noses, ears, 

 and even finger-joints which have been cut off, when they are 

 immediately brought into contact with the exposed surface — 

 a process of growth which like grafting is so far connected 

 witli recrescence that to effect the readhesion new parts must 

 be formed — therefore the last remnant of recrescence in 

 man." 



Vochting has shown that the action of gravity alone is suffi- 

 cient to produce roots on cuttings from plants, and that it is 

 not even necessary to set them in the earth : if twigs of willow 

 bearing; buds of similar age alonsr their whole lenoth are laid 

 in a chamber whicli is kept dark and saturated with moisture 

 (or in moist earth) roots appear on the under side only. Still 

 more striking is the effect of an external stimulus in Lepis- 

 mium radicans, a plant belonging to the Cactus tribe, on the 

 production of roots, for these are produced on any part of the 

 sprouts of this plant from which the light is excluded.^ 



^ Recently Thiersch has even transplanted pieces of the skin of a negro on to a 

 white man, and conversely ; in the first case the pieces soon became white, in 

 the latter black. 



^ For other examples see 0. Weber, Die Gewehserkrankungen im AUgemeinen 

 und ihre Ruckioirkung a/af den Gesammtorganisimis. I. Bd. i. Ahschnitt of Y. 

 Pitha and Billroth, Handhvch der allg. und spec. Chirurgie, 1865. 



^' H. Vochting, Sitzungsh. d. niederrhein. Gesellsch. f. No.tur-u. Heilkunde 

 in Bonn, Sitz. v. 3 Jan. 1876. — On the divisibility of j)lants and the influence 



