INFECTION WITH MACROSPORIUM TOMATO 
79 
fungus grows just as readily on the pulp and extracts of old fruits as on 
those obtained from young tomatoes. Moreover, positive infection has 
been obtained on fruits of all degrees of maturity when the skin is injured 
or removed previous to infection. 
2. Surface sections of old and young fruits failed to reveal the presence 
of stomata or other natural openings in the skin. 
3. As the tomato fruit develops, the surface of the fruit changes from a 
dull to a shiny appearance. The chemical nature of this change has not 
been determined, but sections cut from old and young tomatoes show that 
the cuticle increases in thickness with the age of the fruit. The develop- 
ment of the cuticular layer may be at least a partial explanation of the 
resistance of mature fruit to infection. Dewdrops are more readily retained 
on the dull surfaces than on the shiny and mature surfaces. 
4. The maturity of the fruit as measured by size is correlated with a 
definite resistance of the tomato skin to puncture. The latter may also 
be one of the limiting factors in securing infection with Macrosporium on 
tomato fruit. 
The methods employed in arriving at these conclusions and the detailed 
data obtained in this connection were as follows : 
The tomatoes were grown in a commercial way in the fields of southern 
Florida. The work was limited to one variety, the "Livingston Globe." 
At first fruits of various sizes were selected at random. In order to get a 
more accurate knowledge of the fruit used, a large number of blossoms were 
tagged and pickings for stabbing and inoculations were made from these 
tagged blossoms at the end of each week. In this way it was possible to 
tell exactly the age of the fruit used, from blossoming time until the fruit 
began to show color in the field. 
The Macrosporium cultures used in this work were isolated from tomato 
fruit. They were kept in pure culture and spores obtained according to the 
method described by Kunkel.^ In a few cases spores were taken from fruit 
naturally infected in the field. 
The resistance of the skin of the fruit to puncture was determined by 
the use of the Joly balance as modified by Hawkins and Harvey (/. c). 
The construction of the Joly balance need not be given, but certain modi- 
fications of the balance and the methods followed in calculating the results 
obtained in the use of this apparatus in the present work will not be out of 
place here. 
In using this apparatus for determining the resistance of the tomato 
skin, a fine glass needle 78 microns in diameter fixed to a glass rod with wax 
was suspended from the bottom of the pan. This needle was used through- 
out the experiments except in one instance as indicated. The needle and rod 
were well within the capacity of the spring of the balance. In operation the 
6 Kunkel, L. O. A method of obtaining abundant sporulation in cultures of Macro- 
sporium solani E. & M. Brooklyn Botanic Garden Mem. 1: 306-312. 1918. 
