GRAFT-HYBRIDS AND OTHER CHIMERAS 381 



boundary line between the green and the white tissue of the stem. Hence 

 he concluded that in order to have a periclinal chimera arise from a 

 sectorial chimera the relation between the two kinds of tissue would have 

 to be as shown at 6 in Fig. 158. A bud pushing out at such a point would 

 have an envelope of colorless cells in addition to the colorless epidermis. 



Without doubt this structural principle explains the origin of all 

 natural chimeras. But this principle holds only when there are groups 

 of normally homogeneous tissues in the same stem or bud which have come 

 to differ with respect to one or more characters. In graft-hybrids such 

 diverse tissues come from different plants. The question of the natural 

 origin within the same plant of morphological and physiological differ- 

 ences causing somatic heterogeneity where homogeneity is the ordinary 

 condition is a problem of far greater fundamental importance. It has 

 already been shown that probably all such diversities within single 

 individuals arise as factor mutations (Chap. XIV). 



Other Natural Chimeras. Sectorial chimeras caused by mutations 

 in color factors are the most common natural chimeras. They occur 

 very frequently in citrous fruits, especially in the orange and lemon 

 (see Fig. 161). Other chimeras in these fruits are caused by factor dif- 

 ferences affecting thickness and texture of rind and frequently associated 

 with these are differences in color and flavor of the pulp. Color chimeras 

 are also fairly common in apples and pears and they have been found in 

 grapes, olives and tomatoes, as well as in gladiolus, poppies, sunflowers, 

 dahlias, and doubtless many other flowers. Many valuable variegated 

 forms of ornamental shrubs are mixtures of sectorial and periclinal chi- 

 meras with normal green vegetative parts. Some striking examples are the 

 Variegated Black Elderberry (Sambucus nigra variegata), the Variegated 

 Deeringia (Deeringia celosioides) and the variegated forms of the Japanese 

 Spindle Tree or Strawberry Bush (Euonymus japonicus). Variegated 

 foliage which is caused by factor mutations causing complete or partial 

 chlorophyll reduction are also fairly common among herbaceous plants. 



Two Categories of Variegation. The variegated plants mentioned 

 above, like the white-edged geranium, can be propagated asexually and 

 it is known that in the geranium, snapdragon, four-o'-clock, maize and 

 other plants the variegated character can be transmitted to sexually 

 produced offspring. However, certain variegated plants cannot transmit 

 variegation through the seed although it is transmissible by means of 

 vegetative propagation. Baur has shown that in the latter class, 

 variegation results from a pathological condition and by double working 

 susceptible and immune stocks he determined that it must be caused by 

 a toxin produced by the diseased cells. The long familiar cases of 

 "graft infection" among the Malvaceae are thus explained. It seems 

 that all cases of "infectious chlorosis" in this family can be traced back 



