5§3 
Life-history of Tetraclinis articulata. 
(Fig. 25), of which the outer (exospore) possesses some small and incon- 
spicuous projections. This point is of some interest in comparison with the 
megaspore membrane, as described below. 
A nearly mature pollen -grain is shown in Fig. 26. The nucleus always 
occupies a position to one side of the spore (that next the periphery of the 
old mother-cell wall) and is always surrounded by a single layer of large 
and conspicuous starch grains. 
No further nuclear change takes place till after pollination, the pollen- 
grain being uninucleate when first seen on the nucellus, as in Juniperus , 
Cupressus , and Taxtis (Coker (10), Nichols (36)), and the Callitro'fdeae 
(Saxton (44-47)). When the ripe fresh pollen is mounted in water and at 
once examined, it is seen that the wall is comparatively thin, but after a few 
minutes the exospore usually bursts, owing to the considerable swelling 
which takes place in the endospore, the latter becoming very much thicker 
than it was before. It was found that fixing agents had somewhat the same 
effect on the wall, though to a less extent. Thus an examination of fixed 
material alone leads to erroneous ideas of the thickness of the wall : the 
very thick wall described by the writer in W iddringtonia and Callitris was 
undoubtedly caused partly in this way, though a comparison with similarly 
fixed material of Tetraclinis indicates that the wall is somewhat thicker in 
the former genera, which would probably imply its greater thickness in an 
unswollen condition also. 
Some points may here be discussed in connexion with the meiotic 
divisions. One difficulty, which commonly occurs in the investigation of 
microsporogenesis, was almost absent in the present case, namely, the 
sequence of the different stages. There were here three separate characters, 
which, taken together, made it clear which of any two stages preceded the 
other, at least in the earlier phases, in which alone difficulties of this sort 
arise, (i) The position in the cone. In Tetraclinis the youngest sporangia 
are found always at the apex of the cone, successively older ones appearing 
below until about the last whorl of sporophylls (or sometimes the last 
two whorls), which bear somewhat younger sporangia than those 
immediately above them, but still older than those at the apex. In 
Juniperus , according to Nichols (36), the sporangia at the apex are more 
developed than those at the base, contrary to the condition in Tetraclinis. 
Within a single sporangium some variation occurs, so that the develop- 
ment from the apex to near the base of the cone is not always reliable, 
without supplementary evidence, in fixing the sequence, though it does 
make it possible to distinguish, e. g., stages immediately preceding synapsis 
from those immediately following it. (ii) The degree of separation of the 
mother-cells. In the stage shown in Fig. 1 the spore mother-cells form 
a compact tissue ; shortly after this they begin to show signs of separation, 
the actual separation and rounding out being completed during synapsis. 
