A Dry Rot of the Irish Potato Tuber 
23 
There is a great range in the form of these spores. A spore 
may have an almost straight ventral axis and a dorsal axis with 
a curvature like that of Fusarium sp. (See figure 8, page 38 of 
Appel and Wollenweber 1910 and PL XXIV, figs. 30, 33, 34, 15. 
51, 57, and 60 of the present bulletin), or it may have a ventral 
axis which is straight up to the last septum at the apex and then 
bends in the same manner as does Fusarium solani (PL XXIV, 
fig. 4), and a dorsal axis which is decidedly more humped at the 
two apical septa than is that of Fusarium solani (PL XXIV. fig. 
40; PL XXIII, figs. 3. 4, and 0). 
That the spores can develop a specialized apical and basal 
cell is shown by cultures which were made on potato plugs 
grown at the temperatures 8° to 10° C. Growth was very slow 
under these conditions and the percentage of three-septate 
spores, which ordinarily does not run over 10 per cent, went up 
to 40 per cent. The spores remained attached to the conidiophore 
for a long time, developed slowly, and showed all stages in the 
development of the peculiar structures which we find on the 
apical and basal cells of a spore. 
The apical cells show a tapering such as Fusarium discolor 
and Fusarium rubiginosum do, excepting that the end is a little 
more blunt than in the former (PL XXIII, fig. 18). The basal 
cells show all gradations in the development of the foot or 
"Stiefel." Practically all the types are found here. Hanging 
drop cultures show that the elongation of the apical cell is de- 
termined before it gets to be a conidium, i. e., when the basal cell 
of the preceding conidium is developed. There is a pulling of 
each end much like the pulling of wax, causing the basal end 
of the conidium to taper in the opposite direction. In rapidly- 
developing conidia the cell rounds off quickly and is cut off so 
quickly that these peculiar end structures are not developed. 
This is a predominating condition in the spores of Fusarium 
ttiberivorum. When the medium becomes exhausted or when a 
fungus develops slowly and has an opportunity to mature before 
it germinates, the end cells are developed. 
The origin of the little papillae-like structures which are 
apparent as a heel of the boot has been nicely indicated. Appel 
and Wollenweber 1910 in fig. S, 2, page 38, figured a spore with 
two such papillae. Fusarium didymum shows traces of these, 
and all the others show various stages of degeneration of these 
papillae. When the organism is grown in beef bouillon, frag- 
mentation of these colonies sets in so slowly that the origin 
of these swellings becomes apparent. They are remnants of the 
otter walls which are left as two adjoining spores separate by 
their cross walls. Cells separate last in the middle of the septa, 
