26 $ 
MOTIONS IK PLANTS. - 
when the leaf is first formed or rather developed, it always 
presents its back to the light to contract the muscle, and dry 
the pabulum j but when this is effected it changes its posture 
directly, and never turns again but in a storm of wind j then 
the spiral being more exposed at the face of the leaf than at the 
back, if it retained its usual posture it would be liable to tear 
. the mid-rib to pieces ; but being much contracted by the wind, 
Vinda vioioKt it bends inwardly into so concave a form that the spiral is greatly 
edectoii tJicm. , , , , , , . 
shaded, and not exposed to such extreme agitation : it may 
alw’ays be remarked, that the leaves of the new shoots are almost 
double the size of the old ones ; this is caused by the relaxation 
of the spiral wire while all the pabulum of the leaf is still wet : 
in the elm, I have known it quite double the usual size- la 
shewing the various management of the leaves, I must notice 
that every leaf has not the same mechanical force. There 
appears in all the vegetable tribe a regular scale which rises, 
according to the plant, from the highest to the lowest mechanical 
power ; the sensitive plants are at the top of the scale, then the 
diadelphian tribe, which open and shut their leaves with every va- 
riation of temperature j and so violent are the motions which the 
first frosts of autumn produce on the spiral in the mid-rib of the 
leaf in a cold morning, that they often remain half unclosed the 
whole day, while in the summer they are bent back many degrees 
beyond the horizontal ninety : it is this extreme tension of the 
sjiiral that prepares their decay, still it would not destroy the 
leaf stem did not the top of the oil at the same time render 
aierhanism them more rigid, and liable to break. Ne.xt to the mechanism of 
the diadelphian tribe, may be classed those immense leaves 
that generally belong to pentandrian dygynian plants ; when the 
leaf is very large, it is necessary to give great length and scope t» 
the spiral wire, its force being always proportioned to the 
Jength of space it reaches to effectits motixm — this tribe, there* 
ibre, b-’ve a sort of basket work, (see Fig. 3,) at the lop of the 
leaf, which is twisted into various forms, and through all of 
which ihe spiral wire passes- In all trees and shrubs the mus- 
cles are situated in the last row of the wood, nearest the pith, 
Jjut in all plants that rise each year from the earth, they are 
, found 
