6%. 
“ PHYSIOLOGICAL STIMULI AND ITS SIGNIFICANCE 241 
similarity of these pollengrains and of their content with those men- 
tioned sub 2°., confirms the origin of the first (5°) from the latter 
(2°) ; in most cases the nuclei have divided once completely, so that 
4 daughternuclei are formed; occasionally these 4 nuclei have all 
divided again; frequently not all of them, but 1, 2 or 3 have divided. 
The similarity between the pollengrains sub 2° and 5° is enhanced 
still by the fact that they form not rarely, before anthesis, shorter 
or longer, distended, pollentubes. 
Large, globular, diploid pollengrains. The occurence of a large 
number of the pollengrains mentioned sub 2° and 5° was, as far as 
I could ascertain, in the years 1919, 1921, 1922 and 1923 always ac- 
companied by the presence of a large number of these diploid 
grains. The mode of origin of the latter will consequently probably 
be closely related to that of those enumerated sub 2°. and 5°. It 
therefore seems to me probable that these diploid pollengrains aro- 
se in the following way: the 8 chromosomes of the nucleus of the 
tetradcell or of the young pollengrain have undergone longitudinal 
splitting; before or during anaphase, the physiological stimulus has 
acted as a brake; as a consequence the formation of 2 daughternu- 
clei was impeded, but the mother nucleus became doubled by the 
finished splitting of the chromosomes. Just as mechanical or che- 
mical stimuli, cause the appearance of syndiploid nuclei in somatic 
cells, so physiological stimuli cause the appearance of synhaploid — 
in casu diploid — nuclei with 16 chromosomes in generative cells. 
After this, the diploid generative cell has behaved as a normal ha- 
ploid generative cell. And so we find, as has been told already,just 
as in the normal haploid pollengrain, in the diploid one, a globular 
less deeply staining, but twice as large, vegetative nucleus, which 
has been separated by a spherically curved wall, from an, also twice 
as large, generative nucleus. 
It is however not impossible, that in order to be able to behave in the 
normal way e.g. to reach a stage of rest, a considerable time before 
anthesis, stimuli are necessary, similar to the physiological stimu- 
lus which caused the blocking, just described, of the complete nu- 
clear division. 
The subdivision made, is, as must be mentioned, chiefly based on 
the degree of activity of the generative nuclei before anthesis. Whi- 
le we see, in normal cases, the nuclei enter into a certain phase of 
Genetica V 16 
