July is, 1914 
Fusarium on Sweet Potato 
255 
grown under natural conditions in the open field are considered, it must 
be conceded that the taxonomy of these fungi can be based on studies 
of pure cultures in a normal condition, even when it is impracticable to 
make comparison with material from nature. 
Fungi from nature also contain a great many abnormal stages. It is 
* often very difficult to determine whether the material in hand is normal. 
As to what is normal or what is different from the rule depends in the 
end on the personal judgment of the investigator. 
Before proceeding to a general discussion of the “Criteria of the norm” 
it may be stated that the investigations of the problem have shown that 
species of Fusarium, with and without the perfect form, make a normal 
growth in artificial media. In other words, pure cultures afford an effi¬ 
cient and convenient basis for taxonomic and pathological studies. 
Sterilized plant stems give a “mean proportional” of nutrition on which 
are developed the most characteristic stages without loss of vital power. 
CRITERIA OF THE NORM 
A great variety of conditions are found in pure cultures, especially 
when different media are used, from which it is always necessary to 
select some and reject others. Although sterilized plant stems gave 
ordinarily a good growth, the shape, septation, and relative viability of 
the conidia depended largely on the quantity of water present in the 
culture, whether grown in light or darkness, whether transferred from 
agar or gelatin containing strong acids or alkalis, etc. For instance, in 
Gibberella Saubinetii the cells of spores become barrel-shaped from the 
imbibition of water (PL XIV, figs. H and K , 2) when septate conidia are 
transferred from a concentrated to a dilute medium. 
If, however, the conidia are dried out, a constriction takes place instead, 
with the result that the septa project ringlike (PL XIV, fig. F, seventh 
spore). Under similar influences the ascospores react in the same man¬ 
ner, the cells become swollen (Pl. XIV, fig. E, j), constricted (Pl. XIV, 
fig. E, j) t or remain unchanged (PL XIV, fig. E, 2). Such reactions, 
although more striking in the living cells, are not confined to them, but 
may occur also in killed cells. Vacuoles are often formed in swollen cells, 
in mature chlamydospores (Pl. XIII, fig. £>), conidia (PL XIV, fig. H ), 
in plectenchymata (PL XIV, fig. /), and in overwatered hyphae. Vacu¬ 
oles may occur also in unswollen cells (PL XVI, fig. H). With some 
exceptions, such as in chlamydospores and plectenchymata, the produc¬ 
tion of vacuoles is rarely a characteristic of health and longevity, so that 
it is of doubtful value for the norm, especially when associated with 
swellings. 
There is a stage, however, which is characterized by neither swollen 
nor constricted cells that occurs in conidia, ascospores, hyphae, and more 
or less in other organs of fungi. From a long experience it may be said 
that this stage has the highest constancy in average size, curvature, 
