PERIODICITY OF ELONGATION AND CELL DIVISION. 



RAY C. FRIESNER. 



(^Preliminary Note.) 



For this work seedlings of Cucurhita Fepo, Lupinus alhus, Pisum sativum, 

 Zca eve7'ta, Vicia fuba and Allium Cvpa; and germinating bulbs of Allium 

 Cepa, A. canadcnse and A. cernimm were used. Elongation of the roots was 

 measux'ed by the Barnes horizontal miscroscope fitted with a micrometer scale. 

 For cell division root tips were cut every two hours, killed and fixed in medium 

 chromo-acetic acid, sectioned and stained in Delafield's haematoxylin. The 

 number of cells undergoing division was then detenuined by actual count, and 

 the results reduced to a common area of one square millimeter. All work was 

 done in a dark room at constant temperatures. 



Elongation, when considered in shorter intervals of time (one hour), is 

 found to proceed in a wave-like fashion. These waves are usually of short 

 duration (two or three hours from crest to crest) and show- apparently little 

 uniformity among different individuals in regard to the exact time of high 

 and low points. But in almost all cases these smaller waves are found to be 

 parts of more extensive waves, which appear more clearly when the curves 

 are drawn for longer intervals of time (two hours). Such curves for the 

 elongation of roots grown from bulbs of Allium Cepa fall into two classes. 

 The larger part, about 75%, exhibit three waves of elongation in 24 hours. 

 The remaining 25% shows two waves. Those from seed of Allium Cepa again 

 fall into two classes, which are fairly equally divided, viz., three waves and 

 four waves. An early morning (5 to 9 a. m.) high point is common to all these 

 curves, with the exception of a few of those which show only two waves, the 

 high point being delayed in these latter until near noon. Likewise a low 

 point is found at 9 or 11 p. m. in almost all the curves. The other waves 

 appear with somewhat less uniformity. High points at noon and near sunset 

 and low points in the forenoon and afternoon appear in a large number of 

 curves. 



In the curves for cell division the minor fluctuations are not nearly so 

 prominent, and the waves are much more uniform and regular. Curves for 

 cell division in roots from both bulbs and seeds of Allium Cepa show clearly 

 and definitely three waves in the 24-hour period. Further, the curves are 

 almost identical in regard to the time of high and low points. This greater 

 degree of uniformity and regularity as compared to elongation curves may be 

 explained by the facts, (1) that observations were made only every two hours; 



'This work is being dono .it the lliiversity of Michigan undor the direction of 

 Professor P. C. Newcombe. Complete results will soon be ready for publication. 

 21st Mich. Acad. Sci. Kept., 1919. 



