May 15. 191S Phoma Betae 



171 



It was found, after making a large number of counts, that the maturity 

 and the relative age of the different leaves of a beet plant could be deter- 

 mined by taking an average of the number of stomata on a given area 

 at the base, middle, and apex of the leaf.^ Numerous preliminary deter- 

 minations showed that within certain ranges the number of stomata that 

 occurred on either surface of the leaf was indicative of its age, so, for 

 convenience, all subsequent counts were made on the upper surface. It 

 was ascertained that leaves with 53 to 100 stomata per square milimeter 

 could be considered as mature and were so designated. Every leaf in the 

 outermost whorls on all plants examined gave stomatal counts within 

 this range. Presumably the cells of such leaves had reached their maxi- 

 mum growth and their greatest metabolic activity. Young mature 

 leaves which had a stomatal count per square milimeter of 92 to 1 33 were 

 usually taken from a medium position on the plant and were metabolically 

 active, although they had not as yet reached their greatest size. Leaves 

 which had 134 or more stomata per square milimeter were very imma- 

 ture and were located near the heart growth of the plant. 



In order to determine vv^hich were the most susceptible to infection by 

 Phoma betae, 21 needle lesion inoculations on young leaves, 39 on medium- 

 aged leaves, and 90 on old, mature leaves were made. Only 34 infections 

 developed, and these were on the old, mature leaves. A comparable 

 series of inoculations made with Phyllosticta betae gave 25 infections from 

 91 inoculations on old, mature leaves, no infections from 12 inoculations 

 made on medium-aged leaves, and none from 42 made on young leaves. 

 On the petioles 50 inoculations gave no infection with either organism. 

 The number of infections was not increased when the plants were cov- 

 ered with bell jars or pots, but the infected areas appeared somewhat 

 sooner than on uncovered plants. Typical spots developed, if at all, from 

 two to four days after inoculating; however, this incubation time is in 

 all probability lengthened under less favorable field conditions. (See 

 Table I.) 



1 The stomatoscope originated by Prof. F. E. Lloyd was made available for the work through the kind- 

 ness of the Alabama Polytechnic Institute. At times an adaptation of the stomatoscope with an ordinary 

 microscope was employed. 



