PHENOMENA DUE TO THE TENSION OF TISSUES. 
797 
as may be inferred from the decreasing rapidity with which these tissues contract on 
their isolation, and from the thickening of the cell- wallst The pith from internodes 
of different ages shows on isolation at first an increasing, afterwards a decreasing 
amount of elongation. If the tendency of the pith to expand remained the same 
at all ages, it would, when isolated, elongate more in older than in younger inter- 
nodes, in consequence of the increasing resistance of the tissues which are in a 
state of passive tension ; but when the growth in length has ceased, or soon after, 
the pith loses its tendency to expand, as may be concluded from the fact that on 
isolation from such internodes it elongates less, and finally not at alP, although the 
resistance of the wood has greatly increased ; were the pith now as elastic as before, 
it would expand more rapidly when freed from the very great resistance of the 
wood. 
The following table will now be understood ; the length of the entire internode 
being always placed at loo, and the amount of contraction indicated by negative, of 
expansion by positive percentages. 
Number of the mtemode, 
Change of length of the isolated tissue 
counting from the youngest. 
in percentage of the entire internode. 
Cortex. 
Xylem. Pith. 
Nicoiiana Tabacuni 
I— IV 
-5'9 
-i'5 + 2'9 
V— VII 
-3-1 
-I'l + 3*5 
VIII— IX 
-3*5 
— 1-5 -f 0-9 
X— XI 
-0-5 
— 0*5 -t- 2-4 
do. 
I— II 
— 2-2 
+ 2-3 
III— IV 
— 1*2 
H- 4'2 
V— VII 
— I'O 
+ 2-8 
VIII— IX 
-1-8 
+ 2-7 
Samhucus nigra 
I 
— 2-6 
— 2*6 -f- 4-0 
II 
— 2*0 
-2-8 + 5-5 
III 
-1*5 
— 00 + 1-5 
do. 
I 
-0-6 
+ 3-7 
II 
-1-6 
+ 5-1 
III 
— 0"0 
+ 0-9 
do. 
I 
-i'3 
+ 6-5 
II 
-1-5 
-1- lO'I 
III 
-0-6 
+ 2-3 
These numbers, taken from my Handbook of Experimental Physiology, may 
be supplemented by some others, calculated from the statements of Kraus^ (/. c. 
Table i). 
^ The decrease in the extensibility of the epidermis was determined by Kraus {I. c, tables, 
p. 9), by attaching weights to strips of epidermis. 
^ The relation between the tension of tissues and the state of growth of the internode {i. e. 
the phase of its greatest period of growth) requires fresh and detailed investigation. Kraus's 
Table III (Bot. Zeitg. 1867) shows that the greatest difference of length between cortex and pith 
does not always occur at the time of the greatest growth ; and that even after growth has ceased, 
tensions may still continue. It must however be remarked that the method by which these num- 
bers have been obtained is liable to considerable suspicion. 
^ Kraus has only given the absolute numbers ; but a correct notion can be obtained only by 
comparing them with the length of the internode. 
