Mar. i, 1924 
Morphological Characters of Alternaria mali 
701 
this to be true of the species studied by them, and that the conidia of all 
species of Alternaria are so arranged is generally believed. However, in 
examining published illustrations of Alternaria, even of quite recent date, 
one finds drawings in which the broader end of the conidium occupies 
the distal position. It would be interesting to know whether or not 
such forms really exist. 
At the distal end of each conidium, except quite frequently the last 
one formed in the chain, there is a stalklike projection, variously known 
as the isthmus, beak, or appendage. It is not so deeply colored as the 
conidium and may even be hyaline or nearly so. It may be quite short 
and swollen or long and slender, nonseptate, uniseptate, or even multi- 
septate (fig. 1). It is formed by a growth or a budding of the conidium 
at a time when it is the youngest of the chain, that is, when it occu¬ 
pies the distal position in the chain. From this outgrowth the next 
spore in the chain is developed. In descriptions of species of Alternaria 
the term conidiophore is usually applied only to the outgrowth from 
the conidia-bearing hypha which supports the first formed conidium of 
the chain. In this sense it is considered as the conidiophore for the 
whole chain. The so-called isthmus or beak may also be considered as 
a conidiophore, not in relation to the conidium from which it is an out¬ 
growth and to which it remains attached, but in relation to the distally 
situated conidium which grew out from it. This view is supported by 
the fact that it becomes separated from this conidium as most conidio- 
phores become separated from the conidia which they bear. In one form 
studied the so-called isthmi or beaks tended finally to collapse and disin¬ 
tegrate, which is also a fairly common behavior of conidiophores. A con¬ 
sideration of isthmi as conidiophores also offers an explanation as to why 
the end spores of the chains often have no isthmi or have especially long 
ones. In the former case budding has not started; in the latter the 
long, septate hyphalike structures which may become new conidia and 
conidiophores have been formed. The same sort of budding, but at 
right angles, makes the branches .of the chain. The isthmi or conidio¬ 
phores often do not have as thick walls as the conidia. The tip often 
appears to be swollen and turgid, a condition which facilitates the 
breaking up of the chain through lessening the line of contact between 
the isthmus and the immediately distal conidium. When, however, a 
breaking up of the chain does not occur at this stage, as often happens 
when the chain lies along the surface of the culture medium, it is finally 
brought about by the shrinkage and disintegration of the isthmi. In 
this way, also, they perform a separative function. They are often more 
than one-celled and are incapable of germination. In the case of mature 
conidia the dividing line between conidium and isthmus (conidiophore) 
is easily determined by the position of the convex end of the conidium 
and by the greater width and deeper color of the conidium; in older 
conidia the disintegration of the isthmus may also indicate the dividing 
line. 
TECHNICAL DESCRIPTION 
Alternaria mali.—Realizing that the original description of Alternaria 
mali ( 16 , p . 5<?) was too brief to be of proper use in identification, the 
writer undertook a more careful study of the fungus as it develops natu¬ 
rally on apple leaves, as it appears on apple leaves left in moist chambers 
for several days, and as it grows on artificial culture media. It is hoped 
