STUDIES IN THE DEVELOPMENT OF THE PIPERACEAE 733 
persisted for months in bearing female flowers only. Finally a 
few reduced stamens appeared, and then, after the process was 
once initiated, the same plants continued to bear considerable 
numbers of short-lived, but functional, stamens along with the 
female flowers. Certain of the plants, bearing female flowers 
only, had these accidentally pollinated from a male plant in an- 
other greenhouse. In consequence, they not only formed seeds, 
but, shortly after, began to develop male flowers, and did this 
much earlier than did the unpollinated female plants mentioned 
above. These facts led Strasburger to the conclusion that the 
lack of pollination, and the consequent lack of seed-development 
in this species, leads to a gradual increase of a tendency to ini- 
tiate stamen rudiments, probably b}^ the accumulation of some 
material substance. The same process may be induced even 
more quickly by the influence on the female plant of pollination 
and seed-production. 
The works of Williams ('03) and Hoyt ('07) on Dictyota suggest 
a similar accumulation of some activating substance as the cause 
of the very regular periodic initiation and discharge of the game- 
tangia of this alga. 
In Melandryum rubrum, Strasburger ('00) found that the pres- 
ence of the parasite, Ustilago violaceae, may cause plants that 
have hitherto borne only female flowers to develop staminate 
ones. Goebel ('07), suggests that, in such cases as this of Melan- 
dryum, the course of nutritive substances in the infloresence is 
changed, in consequence of some stimulus produced by the Usti- 
lago. Strasburger ('09^, p. 19) suggests that a substance activating 
male development, and one activating the development of female 
organs, are always present in the infloresence, and that in her- 
maphrodite flowers the two substances come into play separately, 
while in male or female flowers the male substance alone, or the 
female substance alone, completely preponderates. 
Whether the explanations suggested by these unusual types can 
be assumed to indicate the relation of the sex-determinig sub- 
stances in the case of permanently unisexual plants is, however, 
less certain. This is shown, e.g., by the work of Noll on Mar- 
chantia, of which he propagated the unisexual plants by the gemma 
