308 B eer, Development of the pollen gram and antlier of some Onagraceae. 
“rodlet“ layer which is interrupted over the apices of the 
interstitial bodies. 
Düring the later growth of the pollen grain the secondary 
thickening layer has not increased in thickness bnt has, on the 
contrary, become stretched and very mnch thinner than it was 
at an earlier stage. (In Fig. 50 the secondary thickening layer 
has been drawn too thick.) 
In Oenothera longiflora all the pollen grains do not reach 
matnrity but a large proportion of them become arrested in their 
development. They all grow to about 90 g in diameter, when 
their protoplast has become reduced to a hollow shell, but after 
that many of them are unable to continue their development 
owing, no doubt, to the tapetal substance being insufficient for 
the recjuirements of all the pollen grains. 
I liave not given any special attention to the germination 
of the pollen grain but I may mention that the intine of 
Epilobium tetragonum which gives the reactions of both Cellu¬ 
lose and pectose, grows out into a tube which is often branched 
at its free end (Hg. 20). 
The mature pollen grains of Oenothera are bound together 
in long strings by bundles of “hbrils“ which lie between and 
round them. These hbrils are developed front the mucilage 
which, on an earlier page, we saw was derived front the disinte- 
gration of the special-mother-cell walls. These „hbrils“ have 
entirely lost all affinity for callose dyes and have become very 
resistant to solvents. Their properties in many respects resentble 
those of cuticularised structures. 
In the species of Epilobium short bands of the cuticularised 
mucilage bind together the pollen grains which, consequently, 
leave the anther in tetrads. 
Summary and coiiclusions. 
1. In the earliest stages of anther development all the cell- 
membranes contain both cellulose and pectose. The walls of 
the sporogenous cells, however, contain less cellulose than the 
other membranes of the anther. In older anthers the sporo¬ 
genous cell-mentbranes give the reactions of a pectic substance 
alone. 
2. The pollen-mother-cell wall consists of pure callose. Tliis 
substance is formed directly as such by the protoplast and there 
can be no possibility, in the present case, of a transformation of 
cellulose into callose. 
3. In the first and second divisions of the pollen-mother- 
cell seven chromosomes occur whilst in the somatic divisions 
fourteen is the approximate number of chromosomes. The 
presence of two nuclei, one large and the other very small, in 
some quite young pollen grains suggests the occurrence of irre- 
