470 LIFE : OUTLINES OF GENERAL BIOLOGY 



For some grafts it must be admitted that the scion is aifected in 

 a general way by the stock, e.g. as regards the time of flowering; 

 and the stock may be in some measure rein\igorated by the growth 

 of its nobler scion; but the notable point is that each retains its 

 characteristic features. There are, however, some puzzling cases of 

 a different type, which suggest that there may be a mingling of 

 the characters of scion and stock. In other words, there are alleged 

 cases of "graft-hybrids"; and the most famous of these concerns 

 the Common or Yellow Laburnum {Cytisus Laburnum) and the 

 allied Purple Laburnum (C. purpureus). When a scion of the purple 

 sfX'cies is engrafted on the yellow, the resulting growth or a cutting 

 therefrom usually bears purple flowers, as an ordinary graft would 

 do, but along with these there are others that are yellow. In 

 some other ways the plant, which was called Adam's Laburnum 

 (C adami) resembles both the yellow^ and the purple species; and 

 Darwin called it a "graft-hybrid". It interested him greatly, and he 

 speaks of "the extraordinary fact that two distinct species can 

 unite by their cellular tissue, and subsequently produce a plant 

 bearing leaves and sterile flowers intermediate in character between 

 the scion and stock". But, as will be explained later, it seems likely 

 that Adam's Laburnum is even stranger than Darwin supposed. 



A clue has been found by studying some other grafts, such as 

 that effected between the tomato [Solanum lycopersictnn) and the 

 Deadly Nightshade [Solanum nif^rum), the repetition of the 

 technical names being useful to emphasise the fact that the two 

 plants are species of the one genus, Solanum, to which the potato 

 also belongs. Now the shoot that grew from the artificially effected 

 graft combined the characters of both stock and scion. Sometimes 

 the result was more like a tomato, sometimes more like a night- 

 shade, sometimes nearly intermediate between the two. So it looked 

 like a graft-hybrid, and it received the name Solanum tubingense. 

 It is not sterile, as Adam's Laburnum is, and its seedlings revert to 

 one or other of the "parent" forms. 



But some of the results of the grafting of tomato and nightshade 

 look rather different from those to which we have just referred, for 

 one part of the shoot is like the scion and another part is like the 

 stock. This is what is meant by a "chimaera", two living creatures 

 intimately combined, yet each retaining its intrinsic peculiarities. 

 Even when the result of the graft looks very like a blend, a minute 

 analysis may show that here is a sort of patchwork combination of 

 the two components. And if so, it is rather a chimcxra than a graft- 

 hybrid. In fact, it is beginning to be doubtful whether there are any 

 graft-hybrids at all. 



Familiar in gardens and greenhouses are Pelargoniums with white- 

 margined leaves, and some of these are due to grafting and to 

 vegetative propagation from the results of the graft. In this strange 



