14 



DISCOVERY 



side, they are arranged one around the other. Such 

 shoots may, therefore, be termed Periclinal Chimisras. 

 A re-investigation of Cytisiis Ada mi in the hght of this 

 new theor}' has showTi that it, too, as adumbrated by 

 Macfarlane in 1892, is of the nature of a perichnal 

 chimsera, with the core of the Yellow Laburnum and 

 the skin of the Purple Broom. Its tendency to revert 

 to its two constituent components can, therefore, be 

 more readily explained, and indeed it is possible to 

 stimulate it to revert to the ordinary Laburnum by 

 pricking its buds, thus injuring the skin, when the core 

 pushes its way through to heal the wound, and thus the 

 shoot becomes covered by a tissue similar to that 

 constituting its core. 



One feature interesting to note in this connection 

 is that, though Cytisits Adami rarely fruits, its seeds 

 when fertile always give rise, as far as is known, to 

 the ordinary' Laburnum. This can now be explained 

 by the fact that the reproductive cells are formed from 



A BR.\NCH OF ONE OF THE GRAFT-HYBRIDS OF THE 



SIEDLAR AND THE H.^WTHORX. 

 The leaves are less deeply lobed than in Uie Hawthorn, and are 

 hairy like those of the Medlar. 

 B. A BRAXCH OF THE COMMON HAWTHORX, FOR COMPARISON. 



the inner tissues of the plant, and so are entirely those 

 of the Laburnum. Similarly, in Winkler's so-called graft- 

 hybrids the seeds never gave rise to hybrid plants, but 

 only to the plant which formed the core of the chimtera. 



On the Continent it is very common to graft the 

 Medlar on a Hawthorn stock, and in two instances 

 graft-hybrids have been produced from plants grafted 

 in this way. Their rare occurrence is, of course, due to 

 the fact that in most cases such gi-afting is successful, 

 and it is only when the graft fails or subsequently dies 

 that there is a chance of a bud being produced which 

 contains the tissues of both plants. Crafcego-niespilus, 

 as the graft-hybrid has been called, is known in two 

 forms, both of them intermediate between the Haw- 

 thorn and the Medlar, but one resembling the former 

 more closely, %\-hile the other is more like the latter. 

 In this case it is suggested that the form more like the 

 Hawthorn represents a periclinal chimsera with one 

 layer of Medlar cells clothing the Hawthorn core, 

 while in the other form the covering of Medlar cells is 

 more considerable. This explanation seems to fit 

 some of the features exhibited by this interesting plant. 

 Thus, for example, this hybrid's fruits, which in one 

 of the forms resemble in shape and size those of the 

 Hawthorn, are not red like our haws but brown like 

 the Medlar, being clothed with a dark layer of cork 

 characteristic of that fruit. .\lso the seeds of this 

 form, when fertile, produce pure Hawthorns as one 

 would expect in a periclinal chimsera. There are, 

 however, some phenomena which cannot be so easily 

 explained by the chimsera-hypothesis, and the subject 

 is still under investigation. 



Another graft-hybrid has recently been recorded by 

 Professor Daniel, one of the foremost French authorities 

 on grafting. In Brittany it is customaiy to graft 

 certain forms of pears on a quince stock which has less 

 vigorous roots than the pear, and thus keeps the trees 

 in a more dwarf or pyramid condition and also pro- 

 motes earlier and better fruiting. By cutting down to 

 the point of union old plants which had so been grafted. 

 Professor Daniel caused them to produce new shoots 

 near the juncture of stock and scion, and some of these 

 proved intermediate between the two. Though they 

 have been produced some little time ago, they have so 

 far not flowered — a fact which may be taken to 

 suggest that they are true hybrids, or at any rate 

 different from the so-called hybrids mentioned above, 

 which flower freely. An examination of the leaves 

 and stems seems to indicate that they are not peri- 

 clinal chimosras, and it mav well be that we have in 

 this case a more intimate union of the vegetative tissues 

 than in the examples cited above. We may, therefore, 

 take it that, though the chimaera-h\'pothesis of graft- 

 hybrids has cleared up some of the difficulties which 

 have beset the investigators of these curious produc- 

 tions of horticultural practice and experimental skill, 

 the last word on this subject has not yet been said, and 

 that further work on this interesting problem in plant 

 genetics is likely to be fruitful of important discoveries. 



