798 
MECHANICS OF GROWTH. 
Number of the inter- 
node, counting from the ^^^"^e of length of the isolated tissue 
youngest percentage of the entire internode. 
Epidermis. Cortex. Xylem. Pith. ' 
Nicotiana Tabacum III— IV —2-9 — 1'4 +3'5 
V — VI —2-9 —1-3 — 0-8 +2'7 
VII — IX —2-7 —2-1 —o-o +3-4 
X — XII — 1'4 --0-5 — o*o +3*4 
XIII— XV -1-05 -o-o -0-8 (?) +4-0 
Vitis vinifera I — 3'i —1-6 H-6"o 
II — 1"7 —0-0 +8-7 
III -2-5 (?) -i-o(?) +7-1 
IV — o'o — o'o +6-0 
V — Q-Q —0-0 +2-7 
Sambucus nigra I 3-1 —0-0 +o'o 
II ^-1*5 — i-o +6-4 
III -1-6 +6-5 
IV -1-6 +o-3(.?) +6-1 
V — 0-2 +0-2 (?) +0*7 
VI ^-0-5 _ -Q'5 +0-1 
Helianthus tuherosus I — IV —4 '3 -~i'7 +6'8 
V— VI -1-7 -o-o +6-6 
VI — VII —0-9 -0*4 +4-4 
VIII —0-5 -0-0 +3-2 
IX — XI —0-0 +0-9 (?) +2-0 
It is easy to establish the existence of similar contractions of the outer tissues 
and elongations of the parenchyma in the case of growing leaf-stalks, as those of 
Beta, Rheum, Philodendron, &c. 
If a growing internode or a leaf-stalk is split by two longitudinal sections at 
right angles to one another, the parts will bend concavely outwards, evidently in 
consequence of the lengthening of the pith and contraction of the outer tissue. This 
phenomenon is seen most clearly if a thin longitudinal slice is taken from the middle 
of the internode, laid flat, and the pith then halved lengthwise ; as the knife advances 
the two halves will bend concavely outwards. If, instead of cutting the section in 
two, thin strips of tissue are cut proceeding from without inwards, first one including 
the epidermis, next one including the cortical tissue, and finally one including the 
wood, they will all bend concavely outwards, because the adjacent layers are all in a 
state of negative tension on the outside, of positive tension on the inside, and when 
separated, the outer side always becomes shorter, the inner side longer. 
That this bending is caused by simultaneous contraction of the outside and 
lengthening of the inside is at once clear from the measurements already given, but 
may also be observed directly, as will be seen from the following table. Longi- 
tudinal slices of considerable thickness were cut from the middle of growing inter- 
nodes, laid flat, and the pith then halved by a longitudinal cut ; the radius of the 
curvature which each half at once assumed was determined, and the length of the 
convex inner and the concave outer side measured by means of a strip of card 
graduated in millimetres. 
