_]
[Footnote 14: G. Klebs, _Ueber das Verhaeltniss des maennlichen und
weiblichen Geschlechts in der Natur_, p. 19.]
[Footnote 15: Food affords the basis for metabolic changes in the
parent organism, but it is probable that food is less _directly_
related than heat and light to the determination of sex. Sachs, whose
experiments must be given the greatest possible weight, has determined
that the ultra-violet rays of light are necessary to the chemical
changes essential to the formation of the reproductive organs.
(J. Sachs, "Ueber die Wirkung der ultravioletten Strahlen auf die
Bluethenbildung," _Gesammelte Abhandlungen ueber Pflanzen-Physiologie_,
Vol. I, pp. 293ff.) More recently, Klebs has shown that by diminishing
the intensity of light the development of female sex organs in ferns
can be interrupted, so that, in spite of the presence of male organs,
fertilization is impossible; at the same time, the prothallia are
enabled in weak light to grow feebly and to put out small asexual
processes, which in the presence of bright light become normal
prothallia. Similarly, the development of sexual organs in algae
is dependent on a certain intensity of light, and the plant remains
sterile if the light is diminished below a certain point.
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