IN THE DIVIDING ROOT-TIP CELLS OF THE ONION. 23 



absolute durations of the several stages of the Method Chart thus cal- 

 culated approximate so closely the correct values obtained through 

 actual counting and measure, one is justified in concluding that por- 

 tions of 3 waves based upon 16 times as many individual cell-counts, 

 although upon twice as many mitotic stage types, and ^ as many 

 observations, would probably as closely approximate the actual facts. 



The average relative duration of the resting stage in this prelimi- 

 nary work proves to be 66.12 per cent of the entire cycle, when such 

 cycle is conceived to consist of both the resting stage and the 10 mitotic 

 stages, thus crowding the 10 active stages into 33.88 per cent of the 

 11-stage cycle. Consequently, the average absolute duration of the 

 resting stage, during the period sampled, is 336.06 minutes, and 

 that of the entire cycle (including the resting stage and the 10 active 

 stages) is 508.26 minutes,^ which (so far as the number of cells of 

 the region sampled is concerned) means a doubling in about 8 hours, 

 near neither the minimum nor the maximum for such processes. 



A word of explanation is perhaps necessary concerning that chart (No. 

 7) of the preliminary study entitled '' Graphs showing orderly succession 

 of procession indices." This chart is simply another method of show- 

 ing the data tabulated in the Procession Index Table (No. 6) of the 

 same study. The 3 recognizable mitotic waves are traced by the heavy 

 lines connecting successive stages through time-intervals. A heavy 

 line begins at the highest point in the early periods of sampling attained 

 by one of the highest indices of the region. If, by chance, as in wave 1, 

 this happens to be the index for stage 1, at 10^20"^ a. m., the next crest 

 touched must be later than 10*'20'°, and must be that for stage 2, and 

 so on. Thus we connect stages 1, 2, 3, 4, and 5 in one of the straightest 

 lines of the tangle. Wave 2 begins with stage 4, at 10 a. m. This pre- 

 sents a single backward step in that the crest of stage 6 is not quite so 

 far advanced as for stage 5; but, on an average, this line, too, is relatively 

 level. Similarly, wave 3 begins at lO^'lO" a. m. with stage 7, connect- 

 ing the highest point in the region successively for stages 8, 9, and 

 10, in not so level a manner as waves 1 and 2, but still relatively so. 

 Indeed, the comparison of the high points of the mitotic wave to the 

 peaks of a definitely traced mountain range holds good in this first 

 actual study. The procession index corrects the stage indexes through 

 the successive periods of a given mitotic wave strongly in the direction 

 of uniformity, but never completely reaches it. They (the procession 

 indices) are the best available means of unraveling the mitotic tangle 

 in the material used, for if, as in the Method Chart, one attempts in this 

 actual study a similar wave tracing in the chart (No. 5) " Graphs show- 

 ing mitotic and stage indices," he is hopelessly lost. (See pp. 11 and 14.) 



1 If comparison be made with the determinations of the final experiments reported in this 

 paper, account must be taken of the facts that the two experiments differed in temperature, in 

 season of the year, and in variety of onion used (see p. 26) . 



