TENSION OF TISSUES. 



391 



and hence exhibit distinct tensions. If there is a marked dif- 

 ference in the rate of growtli between the internal and the ex- 

 ternal cells in any organ, as is the case in most young stems, 

 the more superficial tissues are stretched to some extent by the 

 internal ones ; hence arise tensions of tissues, the organ in 

 this state being in a balanced condition, in which the equilibrium 

 can be disturbed by slight external or internal causes. The 

 following experiment exhibits the phenomenon of tension very 

 strikinglj' : From a long and thrift}' young internode of grape- 

 vine cut a piece which shall measure exactly one hundred units, 

 for instance, millimeters. From this section, which measures 

 exactly one hundred millimeters, carefullj- separate the epi- 

 dermal structures in strips, and place the strips at once under 

 an inverted glass to prevent drying ; next, separate the pith in 

 a single unbroken piece wholly freed from the ligneous tissue. 

 Finally, remeasure the isolated portions, and compare with the 

 original measure of the internode. There will be found an 

 appreciable shortening of the epidermal tissues and a marked 

 increase in length of the pith.' The young ligneous tissue is 

 generally shortened by its release, but this result is bj^ no 

 means constant. The most astonishing feature is the great 

 difference which exists between tlie length of the external tis- 

 sues and that of the internal tissues which up to the period 

 of isolation they had compressed. The external parts had been 

 plainly stretched to a certain extent, while the internal had 

 been as obviouslj' confined by them. The tensions are not only 

 in the direction of the length, but are also transverse. Similar 

 tensions are to be found also in foliar organs. But there are 



^ The following table exhibits the remarkable differences in tension be- 

 tween the outer and the inner parts of young shoots of Niootiana Tabacum. 

 Each internode is first cut squarely off at both ends, and then carefully sliced 

 lengthwise so as to separate the bark, wood, and pith from each other. Sup- 

 posing the length of the whole internode to be one hundred units, the length 

 of the cortex will fall short of this, while that of the pith will considerably 

 exceed it. 



