Here we have opportunity to observe positive and negative varieties of
the same character. The smaller, and the three-toothed marigold occur
from time to time, provided with ray florets, showing a positive
variation. And the white marigold has produced in our gardens a variety
without rays. Such varieties are quite constant, never returning to the
old species. Positive and negative varieties of this kind are by no
means rare among the compositae.
[132] In systematic works the positive ones are as a rule called
"radiate," and the negative ones "discoid." Discoid forms of the
ordinary camomile, of the daisy, of some asters (_Aster Tripolium_), and
of some centauries have been described. Radiate forms have been observed
in the tansy (_Tanacetum vulgare_), the common horse-weed or Canada
fleabane (_Erigeron canadensis_) and the common groundsel (_Senecio
vulgaris_). Taken broadly the negative varieties seem to be somewhat
more numerous than the positive ones, but it is very difficult to come
to a definite conclusion on this point.
Quite the contrary is the case with regard to the color-varieties of red
and blue flowers. Here the loss of color is so common that every one
could give long lists of examples of it. Linnaeus himself supposed that
no blue or red-colored wild species would be without a white variety.
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