88 AN INTRODUCTION TO MODERN GENETICS 



one species covered by a layer, which may be one, two, or more cells 

 thick, of the other species (periclinal chimaeras). 



Similar mosaic parts may be produced by a somatic elimination or a 

 mutation (factorial or chromosomal) in a growing point, and consist in 

 such cases not of mosaics between different species, but between 

 different mutant types of the same species. Thus we may get (e.g. from 

 callus tissue formed on the cut back stems of Solanum) pericUnal 

 chimaeras with a core of tetraploid tissue in a skin of normal diploid. 

 The most famous of the inter-specific hybrid chimaeras are perhaps 

 the Bronvaux Medlars, whose nature was for a long time in doubt but 

 which are probably periclinal chimaeras with one or more layers of 

 Crataegus mespilus over a core of C. monogyna; another is Cytisus adami 

 which has a skin of C purpurus and a core of C. laburnum. The morpho- 

 logical characters of such hybrids usually show the effects of both sorts 

 of tissue, giving the impression that each specific type has retained its 

 own characteristics in so far as the mechanical conditions imposed by 

 the neighbouring tissue will permit. A very common type of chimaera 

 is between normal tissues and tissues (of the same species) lacking 

 chlorophyll; many green and white variegated plants are of this type. 



