203 
Beer. — St tidies in Spore Development. 
powers of the microscope and in the most delicate microtome sections. In 
Fig. 4 a young pollen-grain is represented lying within the special wall. 
Here the pollen-protoplast has contracted under the influence of the re- 
agents, and the young exine has also separated from the special wall from 
the same cause. Under these circumstances the newly developed exine 
can be seen exceptionally well as an independent membrane of great tenuity. 
In somewhat older pollen-grains a structural differentiation of the exine can 
be detected which even at this early stage exhibits some complexity. The 
exine can now be seen to consist of an outer lamella, upon the inner face 
of which is deposited a network of thickening bands. At the angles of the 
meshes of this network the rudiments of the future spines already occur. 
Between the thickening bands and the outer lamella a narrow cleft or 
unstained space can be seen, and this is the position in which the rodlets of 
older pollen-grains are developed. 
Fig. 5 shows the inner face of the exine at this stage in surface view. 
The more deeply staining system of thickening bands is seen to form 
a reticulum with polygonal (mostly hexagonal) meshes upon the lighter 
outer layer of the exine. At the angles of the network the spine rudiments 
are seen as deeply coloured dots. 
Fig. 6 c represents the same stage in section. Here the alternation of 
thicker areas, where the thickening bands lie, with thinner intervals is seen. 
In very delicate microtome sections the separation of the thickening band 
from the outer lamella of the exine by a clear, unstained space or layer 
can readily be made out (Fig. 6 b). 
Where the section has passed through the spine rudiments the appear- 
ance is somewhat different. In Fig. 6 a it will be seen that the thickening 
band appears to be pushed inwards (by the colourless layer) at each spine 
rudiment so as to form an internal spine. The spine rudiment itself appears 
as a deeply stained particle just within the apex of each of these projections. 
The external surface of the exine is still completely flat and smooth. 
The mode of development which these component parts of the exine 
follow is a difficult matter to decide with certainty. There can be little 
doubt, I think, that the outermost lamella and the system of thickening 
bands are successive developments secreted by the pollen-protoplast one 
after the other. The thin structureless membrane of such stages as that 
represented in Fig. 4 I believe to correspond to the outermost lamella 
alone. Upon this the bands of thickening are laid down by the protoplast 
in somewhat older pollen-grains. At first these bands are so thin and 
faintly marked as to appear as little more than shadowy traces upon the 
inner face of the membrane, but they rapidly gain in distinctness as develop- 
ment proceeds and new material is added to them by the protoplast. 
Exactly how and when the spine rudiments and the rodlets are first developed 
is a more difficult problem to determine. The impression which I have 
