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 bin 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. Somestriking examples are the 
Variegated Black Elderberry (Sambucus nigra variegata), the Variegated 
Deeringia (Deeringia celostoides) and the variegated forms of the Japanese 
Spindle Tree or Strawberry Bush (EHuonymus 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 
“oraft infection’? among the Malvacew are thus explained. It seems 
that all cases of ‘‘infectious chlorosis” in this family can be traced back 
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