586 
Journal of Agricultural Research voi. xxvm, No. 6 
Table I .—Spores of Physalospora malorum —Continued 
ARRANGED ACCORDING TO RATIO OF LENGTH TO WIDTH 
Ratio (microns) 
Total 
1.5 
2 
2.5 
3 
3.5 
4 
4.5 
5 
Ascospores on apple._. 
223 
6 
46 
77 
58 
21 
11 
4 
Ascospores on currant___ 
146 
1 
27 
59 
40 
13 
5 
1 
Pycnospores in culture from ascospores on 
apple___ __ 
100 
3 
42 
51 
4 
Pycnospores in culture from ascospores on cur¬ 
rant - 
100 
8 
41 
46 
3 
2 
RELATION OF SIZE AND STRUCTURE OF SPOROCARPS TO HOST 
The only striking difference in the fungi on the two hosts is in the size and 
structure of the sporocarps and this the writer believes to be due to the effect 
of the substratum. Plate 1, A, shows in vertical section a typical mature peri- 
thecium of Physalospora malorum on apple, while Plate 1, B and C, show mature 
perithecia of P. malorum on currant. Usually on currant several perithecia are 
grouped together in a single sporocarp, as shown in Plate 1, B, and these are 
surrounded by and supported on a mass of stromatic tissue. Even when, as 
occasionally happens, a perithecium occurs singly on currant it will almost 
invariably be partly embedded in a mass of stromatic tissue. 
A similar difference is found in the pycnidia. On apple the pycnidia usually 
occur singly and without any noticeable amount of stromatic tissue (Plate 1, D), 
whereas on currant the pycnidial cavities are usually grouped in large stromata 
(Plate 1, G, and Plate 2, A), or when they do occur singly are partly surrounded 
by stromatic tissue (Plate 1, F). 
The presence or absence of a stroma has been considered of sufficient impor¬ 
tance to separate species or even genera and families. That it can not be so 
considered in the case now under consideration is abundantly proven by the 
fact that the presence of a stroma in the fungus on currant is not a constant 
character. If a pure culture of the fungus from currant is permitted to fruit 
on a portion of sterile apple twig, the resulting pycnidia will mostly occur singly 
and will have little, if any, associated stromatic tissue. The same thing is true 
if such a culture fruits on any twig having a smooth bark with fine texture. 
Plate 1, E, for example, shows a typical pycnidium of the currant fungus grown 
in pure culture on a sterile portion of a black raspberry cane (Rubus). On the 
other hand, when the fungus from apple is grown on currant the condition is 
reversed, and the sporocarps are large and usually contain several spore cavities. 
The experimental evidence that the size of the sporocarp is directly influenced 
by the host has reference at present merely to the pycnidial stage because we are 
not yet able to produce ascospores in quantity in culture. There is no reason 
to suppose, however, that the same relation would not hold for the perfect stage. 
In nature ascospores and pycnospores occur in similar sporocarps and on currant 
were found to occur in the same sporocarp. 
Attention has already been called to the fact that the amount of stromatic 
tissue about pycnidia and perithecia of Botryosphaeria ribis is directly affected 
by the host upon which it grows. 10 In the case of this fungus as in Physalospora 
a larger ascocarp is formed when the fungus fruits on currant than when it fruits 
on twigs having a thinner and more closely knit bark. Whether this difference 
jo Stevens, N. E., and Jenkins, A. E. occurrence of the currant cane blight fungus on other 
hosts. Jour. Agr. Research 27: 837-844, ilus. 1924. 
