Mar. 8,1924 Aecidiospore Discharge in Relation to Spore Wall 
75i 
levels so that no two spores have exactly the same shape as they exist 
under these pressures. Only one layer can be made out in the wall of 
the youngest spore in the chain. Beginning with about the fourth 
spore from the base, differentiation of the wall into three distinct 
layers can be observed. The outer and inner layers are very thin, 
while the middle layer is somewhat thickened. Pore formation begins 
at about this stage. All of the spore wall takes a deep orange G stain 
except that region where the pore will later be found. This part becomes 
considerably thickened, remains slightly separated from the rest of the 
“ exospore ” and takes a brilliant gentian violet stain; the older the 
spore the larger and more easily distinguished becomes lie violet-stained 
portion, which for convenience may be referred to as a “ plug.” It is very 
much flattened (fig. 3); its thickness is only about half its diameter. 
Viewed edgewise it appears to be made up of striae similar to the lines 
in the walls of the peridial cells. At least it is not homogeneous in its 
structure. Viewed flatwise it seems to be composed of wedge-shaped 
segments placed edge to 
edge. The prevailing 
shape of the fully formed 
plug is that of a basin or 
a hatter's block (fig. 3, 
B). The writer's con¬ 
ception of the mechanics 
of spore discharge fol¬ 
lows. 
Theaecidium is slightly 
contracted where it comes 
in contact with the lower 
epidermis of the leaf, or 
if not, the effect is the 
same because the epi¬ 
dermis offers more resist¬ 
ance to the forces of ex¬ 
pansion developed by 
the growing sorus than 
do the cells of the meso- 
phyll. The spore walls 
are highly elastic so that 
at the spot opposite each 
plug the adjacent wall is much indented (fig. 3, A). As the maturing 
spores are being pushed upward new adjustments are brought about as 
the spores slide by one another over the separating plugs. While the 
shape of a spore under pressure is very irregular, it loses its angularity 
as soon as it is forced past the level of the epidermis so that these 
confining pressures are removed. The parts indented by the plugs 
spring back into place. The reaction of the walls of other spores touch¬ 
ing this one is such as to violently expel the spore from the cup. 
A section of a young aecidium is shown in figure 2, the layers of cells 
surrounding the aecidium being shown diagrammatically. The outlines 
of peridial cells, spores, and pore plugs were traced with the aid of a camera 
lucida. The unshaded spores in the upper part of the aecidium have 
no cytoplasmic contents. They are degenerating and by so doing may 
supply mucilaginous substances which by swelling assist in rupturing the 
Fig. 2.— G. myricatum. Section of young aecidium. 
further explanation.) 
(See text for 
