192 Melanose and Stem-End Rot [390 
planation of these geographical limitations may be found. 
Such problems as this are worthy of serious attention. Some 
of the logical possibilities of this particular case may be 
mentioned, by way of preparing for further observations and 
for constructive experimentation. 
One possibility that always presents itself in connection 
with a limited geographical distribution of any parasite is 
that sufficient opportunity or time may not yet have been 
afforded for the parasite to become distributed throughout 
the area occupied by the host. But this possibility seems not 
to apply in the present case. As has been mentioned, after 
melanose had become common in central peninsular Florida 
there took place a free interchange of many kinds of citras 
nursery stock between Florida, on the one hand, and Cali- 
fornia and Cuba, on the other. Many carloads of young 
citrus trees were shipped from nurseries located in the 
Florida area where this Phomopsis was most virulent, no 
effective quarantine regulations were in operation at that 
time, and it is impossible that the fungus has not long since 
been thoroughly distributed. All or nearly all of the citrus 
varieties grown in Florida have been planted, at one time 
or another, in California, and the recent and extensive 
Cuban plantings have been made with nursery stock from 
Florida. 
Of course, climatic conditions may furnish an explanation 
of the facts here dealt with, but the climatic relations of a 
fungus like Phomopsis citri are probably even more complex 
than are those of higher plants. For the growth of such a 
parasite it is not only necessary that the climatic conditions be 
suitable for this organism, but it is also essential that the com- 
plex of these conditions be naturally so arranged or balanced 
that the host-plant may be in just the proper state to favor 
the virulent development of the parasite. The time factor 
is especially important in the process of infection; it must 
happen that the host is in a condition to be readily infected 
just at the time when the fungus spores reach it. 
