Mar. 3,1923 
Gray Mold of Castor Bean 
705 
rial fixed 24 hours after inoculation showed abundant spore germination 
and early stages of penetration before there was any indication of injury 
to the parts inoculated. Actual penetration of the cuticle was accom¬ 
plished, as in the case of B. drierea, by a fine peglike projection from the 
point of contact of the developing germ tube. Plate 13, A to H, shows 
camera-lucida drawings of 5 to 7 n sections of castor bean leaves, showing 
hyphae penetrating the cuticle. No attempt was made to demonstrate 
muscilaginous substance at the point of contact of the germ tube. After 
penetration of the cuticle was accomplished, the fungus advanced quickly 
into the host, and breakdown and disorganization of the host tissue was 
rapid and complete. The cuticle remained a barrier to easy penetration 
of other hyphae for some time, however. Sections of materials fixed 48 
hours after inoculation showed a tangled web of germ tubes, with abund¬ 
ant production of appressoria and distinct indentations below many of 
them, indicating marked mechanical pressure. Plate 9, A and B, shows 
such indentations. The disorganization of the host tissue beneath, with 
consequent loss of turgor, has permitted deeper indentation than occurs 
with initial penetration. The tangled web of hyphae of germinating 
spores which is very evident in the figures would seem to be in itself a 
factor in withstanding the backward pressure exerted by a penetrating 
tip. These sections were very thick, approximately 25 ju, and were 
photographed to show the abundance of hyphae and their apparent 
pressure on the host, in this case a male blossom bud. Plate 9, C, shows 
hyphae of the fungus within such a male blossom bud and the complete 
disorganization of host tissue. 
Sections of nectaries from a castor-bean inflorescence showed the pro¬ 
tective cuticle to be continuous over its entire surface. The cuticle 
appeared in some cases to be even thicker over the nectary than over the 
surrounding parts of the host, but there is no evidence for a conclusion 
that this had any influence on the comparative freedom of nectaries from 
primary infections. 
The fact that the seeds are important carriers of the pathogene has 
already been mentioned. Many observations were made of the presence 
of the mold on and within the seed. Plate 7, B and C, shows the mold 
itself growing on the caruncle of castor bean seed. The mold was often 
seen growing over immature pods and penetrating them. Such materials 
were fixed and embedded, and the presence of hyphae of the fungus was 
demonstrated with differential stains. Since such seed may reach the 
size and appearance of maturity, it is evident that the fungus may be 
carried by this means, along with healthy seed. 
HOST LIMITATIONS OF THE FUNGUS 
One of the first considerations, when it was determined that the gray 
mold was a new organism in America, was to determine whether or not it 
might prove to be a danger to other crops. At Cornell University, in 
January, 1919, a number of plants that were available were inoculated 
under exactly the same conditions that were given the castor beans 
inoculated successfully at the same time. The common geranium 
{Pelargonium sp.) was more severely infected than any of the others. 
The growing point of the plant was entirely killed, and several of the 
younger leaves were attacked at their margin (PI. 8, C). The organism 
continued to spread as long as moisture conditions were kept favorable. 
Reisolations were made and identified. As soon as the moist inoculation 
