PHYSIOLOGICAL STIMULI AND ITS SIGNIFICANCE 299 
und dies musste auch durch Modification der ausseren Lebensbedingun- 
gen (z. B. niedere Temperatur, Ernahrungsmangel etc.) verursacht 
worden sein, weil es nicht unwahrscheinlich ist, dass in diesen Ver- 
suchspflanzen solche Unregelmässigheiten fur gewöhnlich vor sich 
geben’. We perfectly agree with this opinion of SAKAMURA. GATES and 
REES (1921) are of opinion, that the occurrence of several globular nu- 
clei, in the tetrad cells already, can be ascribed to another cause still: 
„Binucleate pollen mothercells were found both before, during, and 
after synapsis. It seems probable that such cells arise by the breaking 
down of an incomplete wall between two cells in the archesporial stage 
(l.c. p. 394 and fig. 69). 
/. SAKAMURA did not succeed however to find good, ripe pollen- 
grains (cf. his résumé point 12: „Aber es ist mir nicht gelungen, diese 
Pollenkörner bei weiterm gesunden Wachstum zu erhalten). The rea- 
son of this will probably be, that in stead of chemical or physical stimuli 
physiological stimuli must act, while the flowerbud is intact and attached 
to the plant, as in our experiments. 
g. Whether it will ever be possible, to regulate the external condi- 
tions in such a way, that exclusively diploid or plurinuclear pollengrains 
are formed is a question, which cannot be answered. 
It is certain that the susceptibility of different varieties towards 
the same stimulus differs greatly. Comparitive experiments have pro- 
ved this already. Moreover slight differences in physiological stimula- 
tion can be accompanied by a modification in the proportion of normal 
pollengrains. (Cf. what has been said of Yellow Hammer abn. W. and 
abn. N and the descriptions of their pollen). Exact knowledge of and per- 
fect control of these physiological stimuli may largely influence the bree- 
ding-methods of the future. 
Goldschmidt (1923) is sceptical in regard to the significance of the du- 
plication of generative nuclei in matters of evolution. Da ‘sel zu: 
nächst der Typus der chromosomalen Mutation vorausgenommen. Es 
kann wohl keinem Zweifel unterliegen, dass durch Verdoppelung eines 
oder aller Chromosomen auch in der Natur Formen entstehen können, 
die beständig bleiben, also etwa gigas-Formen oder Formen, die BLA- 
KESLEE’s Datura-Mutanten analog sind. Es ist aber schwer vorstellbar, 
dass ein solcher Vorgang durch öftere Wiederholung zu weit-gehender 
Formensonderung führt und zur Differenzierung von Arten”. However 
this may be, the importance for practical cultivators can not be denied. 
