I 7 4 THE CAUSE OF SPECIFIC SHAPE 



development of the conducting channels at the point of junction would 

 have the same effect as a partial local interruption of these channels in 

 the ungrafted plant. If the graft is only capable of limited growth and 

 development, the root-system of the stock may experience a corresponding 

 decrease in its growth, and vice versa. Thus a pear grafted on a quince 

 stock develops a smaller stem than usual owing to the root-system of a 

 quince being less developed than that of a pear. 



The graft and stock mutually influence one another, though usually 

 not to any greater degree than do the corresponding organs of the original 

 plant. The cultivated varieties of apples, gooseberries, and roses preserve 

 their identity as grafts upon different stocks, and this even when several 

 different grafts are transplanted on to the same stock. Similarly potato- 

 tubers develop as usual when shoots of the thorn-apple (Datura) are 

 grafted upon the root-stock in place of the potato-shoot. 



Nevertheless in some cases changes take place in the colour, taste, or 

 shape of particular organs of one or of both symbionts. For instance, the 

 variegation or albinism of the leaves of a stock of Abutilon Thompsoni is 

 often transferred to a graft of this plant having normal leaves. Similarly 

 variegated foliage appeared upon Althaea officinalis when a graft was 

 attached to an albinic stock of Abutilon Thompsoni. Further, green potatoes 

 become violet when shoots of this colour are grafted upon them 1 . In 

 general the results produced are much less prominent than in natural 

 symbiosis. It must, however, be remembered that grafting is only possible 

 in related plants, and for the most part the stimulating interactions need 

 to act across a considerable distance, so that they can only with difficulty 

 overcome the influence exercised upon the growing tissues by those of 

 the same plant immediately abutting upon them. It is, however, possible 

 that in spite of this, reactions may be found to occur in widely removed parts, 

 and this might readily be produced by the agency of some stimulating 

 metabolic product. That metabolic products may pass from one symbiont 

 to another is obvious, and is, for example, shown by the fact that potato- 

 tubers accumulate a certain amount of atropin when the aerial shoot is 

 replaced by a graft of Datura Stramonium 2 . 



The spot- disease of tobacco leaves seems in fact to be caused by some 

 metabolic product or products, and hence the disease is able to spread from 

 a stock to a graft upon it. Beyerinck 3 in fact found that the disease could 



1 Lindemuth, Landw. Jahrb., 1878, Bd. vii, p. 887; Gartenflora, 1897, p. i; Vochting, 

 L c., p. 92. 



* Strasburger, Ber. d. Bot. Ges., 1885, p. 39. 



3 Beyerinck, Ueber ein Contagium vivum fluidum als Ursache der Fleckenkrankheit der Tabaks- 

 blatter, 1898 (repr. from Verb. d. Kon. Akad. d. Wetenschappen te Amsterdam). It is, however, 

 still possible that the disease is due to infection by a micro-organism, for filtration through porcelain 

 under pressure is not an absolutely sure mode of sterilization. Cf. Iwanowski, Centralbl. f. Bact., 

 1901, 2. Abt., Bd. vii, p. 148. 



