PHYSIOLOGICAL STIMULI AND ITS SIGNIFICANCE 229 
During the second half of January 1921 one specimen of Yellow Ham- 
mer. abn. N. and one of Yellow Hammer abn. W. were taken from the 
soil and forced into flower. For this purpose they were placed in a 
glasshouse with a temperature oscillating between 62° and 77° F. The 
grade of humidity was high in that house. On the 3d of February the lo- 
wer flowers of the inflorescence had fully expanded within the protec- 
ting bags. 
In the same glasshouse there were, during the first half of January, 
10 specimens of the variety Gertrude each of which produced one inflo- 
rescence (Gertrude abn. N.). They had been bred by the firm P. NELIS 
and Son and had been submitted to the same treatment as those of 
Yellow Hammer abn. N.; more over these plants had been forced into 
early flowering by the firm mentioned, which sent them to me, just 
before the flowers began to expand. During flowering, the temperature 
oscillated in the glasshouse between 59° and 73.5° F. 
The two specimens of Homerus, a diploid variety, by its nature an 
early flowering one, were also placed in the hothouse. 
The two varieties mentioned in Chapter I, the chromosomenumber 
of which has been determined, and the varieties Gertrude, Yellow 
Hammer and Homerus, mentioned in Chapter II have been more 
fully described in the publications 1920 b and 1921 a of the list of 
litterature. 
III. INVESTIGATION OF THE HABIT OF THE LIVING POLLENGRAINS; 
GERMINATION EXPERIMENTS 
a. The different conditions under which the pollen can occur. 
In the communication, sub 19210 in the list of litterature, attention 
was called to the fact that diverse transitions may exist between nor- 
mal and abnormal pollen. If we care to distinguish initial — interme- 
diate — and final stages these might be characterised as follows: 
1°. the pollen is normal in shape; fertile and sterile grains are present ; 
the latter are large and ellipsoidal, they make the impression of ha- 
ving died at a late stage of development. The percentage of sterile 
grains, depends, as has been said already, on the variety used; in 
the case of the varieties Gertrude, Yellow Hammer and Homerus e.g. 
those with which we experimented, it remains below 10 WAL 
2°. besides large ellipsoidal sterile pollengrains, smaller ellipsoidal and 
