SAPROPHYTISM AND SYMBIOSIS 779 



another species or variety, though it is held generally that readily diffusi- 

 ble organic substances may thus migrate, in addition to water and in- 

 organic solutes. For example, in Abutilon, albescence may develop 

 in a green-leaved scion on an albescent stock. The migration of such 

 alkaloids as atropin and nicotin from one graft -symbiont to the other 

 may now be regarded as demonstrated. For example, atropin may 

 accumulate in the potato or in the tomato when intergrafted with Atropa 

 or Datura, and nicotin passes from the tobacco plant (Nicotiana Taba- 

 cum) into Nicotiana alata and also into the potato. In all of these cases 

 the alkaloids migrate into the other symbiont, whether the alkaloid- 

 producing species is used as stock or as scion. However, attempts to 

 induce the migration of hydrocyanic glucosids between stock and scion 

 in Phaseolus have met with no success. In a number of cases the form 

 of a plant may be changed by grafting, the pear, for example, becoming 

 dwarfed when grafted on the quince; some varieties of the apple, when 

 used as scions, exhibit changes in the compactness of their branching. 

 In the grape, grafting has been found to cause the modification of 

 many characters, such as the size of the vine, the form of the leaf, 

 the size of the seeds and of the fruits, and the juiciness of the fruits. 

 French vineyards have been saved from the ravages of the destruc- 

 tive plant-louse, Phylloxera, by grafting the vines on immune American 

 stocks. 



Graft hybrids and chimeras. It has often been supposed that the 

 stock and scion sometimes fuse in such a way as to produce new shoots 

 that are intermediate between the two graft symbionts; such new forms 

 have been termed graft hybrids. Famous cases of supposed graft hybrids 

 are : the Bizzaria orange, which is thought to have arisen from a graft 

 of Citrus Aurantium and C. medica; Cytisus Adami, which is thought 

 to have arisen from a scion of C. purpureus on C. Laburnum; and 

 Crataegomespilus, which is supposed to be a graft hybrid between 

 Crataegus monogyna and Mespilus germanica. A case has been reported 

 where there arose shoots of intermediate character when a pear scion 

 was grafted on a quince stock. Usually seedlings from these " graft 

 hybrids " revert to one or the other of the two parent forms, but in at 

 least one instance, progeny of intermediate character has been reported ; 

 for example, if white beets are grafted on red beets, about a fourth of 

 the progeny of the white scion is red or reddish. Recently a remark- 

 able fusion of the stock and the scion has been produced by a graft 

 between the tomato (Solanum Lycopersicum) and the nightshade (S. 



