300 Report of the Botanical Department of the 



Kraus^ measured at various heights the bark tensions of many 

 trees, shrubs and herbaceous plants by isolating rings of bark and 

 replacing them to find the amount of contraction that had resulted. 

 Although the method gives neither quantitative nor even accurate 

 comparative results, it shows beyond question that there are regions 

 of maximal and minimal transverse bark tension at different heights 

 on a trunk. 



In a one-year-old shoot or stem the transverse tension between the 

 bark and wood was found to increase from the tip downward so that 

 the basal internode had the greatest bark tension. As growth con- 

 tinues the tension increases up to a certain point and then the bark 

 cells divide and grow, and thereby reduce the tension until further 

 growth increases it again. This process is said to begin at the oldest 

 internodes and to pass gradually towards the distal end. 



In some measurements of transverse tension taken during winter 

 considerable variation was found in the distribution of maxima and 

 minima on the stems and branches of trees, but it usually began with 

 zero at the distal end and increased to the maximum on the largest 

 branches before reaching the main crotches and afterwards increased 

 to a second maximum somewhere about the crotches. In some in- 

 stances the bark tension of stems decreased towards the ground and 

 in others it was found to increase to another maximum at the surface 

 of the ground or about the upper roots. Kraus holds that transverse 

 tension develops in the bark and gradually increases to a maximum 

 because the bark lags more and more in growth until at a certain 

 stage in its life history its outer portion ceases growth and is ruptured, 

 resulting in the roughened bark typical for the species. The cortical 

 parenchyma seems to be tardiest in tangential growth during the 

 first few years and therefore suffers the greatest transverse or tan- 

 gential strain, until after a certain number of years it reaches a maxi- 

 mum which is followed by a more rapid growth and a consequent 

 reduction of the tension. 



It is thus seen that transverse bark tension has seasonal and life 

 history maxima and minima which may change their positions up 

 and down tree trunks and branches. It was also found that there 

 is a daily periodicity in the tension with a maximum at night and a 

 minimum about 2 p. m. It is held that the daily periodicity cannot 

 be due to root pressure nor to differences in transpiration because cut 

 branches immersed in water still exhibited the same periodicity. 

 Neither do temperature variations appear to affect the daily period- 

 icity as long as they do not go below vegetative requirements. But 

 when a branch was placed in the dark about noon (the period of mini- 

 mum tension) the maximum was reached in one or two hours and 

 remain-ed so till exposed to light, when the normal periodicity was 

 again resumed. 



' G. Kraus. Die Gewebespannung des Stammes und ihre Folgen. 

 Bot. Zeit. 25:105-19; 121-26; 129-33; 137-42. 1867. 



