-Oétob. 
extremely near ; 
only by» Contact; 
Leaves of Plants by Light, 
pacted Bundle, ftrait forwatd’to the Extremity 
of the Rib; and theré, “as there is no ‘odd 
‘Lobe to clofe the Leaf, ‘they terminate in a 
Point, covered by the common Integuments. 
From each Side of this middle Rib rife ‘the jy: 
‘Footftalk$ of the feparate Lobes. "Thefe are? 
formed of a Multitude of delicate Veffels, ranged | 
‘clofe together, and are confined by the Covering, 
which is the common Rind of the Plant ea 
tinued to that Part. 
At the Bafe of each Lobe there is another 
complex Clufter of Fibres: from this Part 
they are protended forward, ftrait to the End 
of the Lobe; and they fend out only flicht oka 
ches into the feveral Parts of the Leaf — 
‘This is the particular Structure of the Abrus. 
Leaf, as feen upon a careful Diffection, and with 
it agrees with the general 
‘Conftruction we have given before, as the com- 
a good Microfcope : 
mon Courfe of Nature in thefe Parts; and it. 
will regularly explain the Change of Pofture | 
in the Lobes, under the different Influence » 
of Light. 
Light is fubtile, active, ad penetrating : by, 
the Smilies of its conflituent Parts, it is) 
capable of entering Bodies; and by the Vio- 
lence of its Motion, of producing great Effects 
and Changes in them. Thefe are not perma- 
nent, becaufe thofe Rays which occafion them 
are, in that Action, extineuifhed, and loft. 
Bodies may act on Light without Conta; 
for the Rays will be refle€ted when they come 
but Light can a& on Bodies 
nid in that Contaé& the 
Rays are loft: — | 
The Change produced in the Pofition of the 
is the Refult of 
a Motion occafioned by its Rays among their 
Fibres: to excite this Motion, the Tighe mult 
touch thofe Fibres; and where Light touches, 
at adheres, and becomes immediately extin- 
guifhed. 
 Thefe are the everlafting and invariable Pro- 
perties of Light: and, according to’ thefe, the 
Change we attribute to it, being once effected, 
muft be continued as naturally and as neceffarily 
as it began, fo long as the Light continues ; and 
no longer. 
The ‘raifing of the: Roti: in aia Didaots 
will be owing to the Power  of..thofe Rays 
which at any one Inftant fall upon them: thefe 
become extinguithed , but others immediately, | 
fucceed to them, fo long as the Air‘in which 
the Plant ftands, i is enlightened. It ought: there- 
fore to be’ feen, that in full Light, the Lobes 
continue in their moft. ‘raifed Pofition 5 and. 
that they droop "from ‘that in Proportion as 
the Light becomes a eg 
oa 
This which appears neceflary from the Bowers 
of Light, ahd the Conitruction of Hi tisho is true! 
alfo in F act. 
We have feen that the Footftalks': wre thefe 
Lobes, are, Clufters. of F ibres protended -from 
the Center of the Stem; that they are con- 
N°. 59. 
From this‘Part they aré carried inafmall com-. | 
® 
tinted through the Lobes ; 
port them ' 
‘in “theit™ Pofition, | “wharfoever'’ 
is. - at ae 4 si sacee f Pd 
The Effe& of ' Light upon ‘thefé ‘Fibres is, 
the ‘putting: them into ‘an inceflant~ Vibration : 
this happens neceffarily from ‘the continual ‘Im- 
pulfion and Extihion of the Corpufcles, of 
which Light’ is compofed, and the frefh Im- 
pulfion of others, ‘upon the Extinction of the 
irae 
It cannot be, Bue ‘that a Clufter of delicate 
Fibres, affected inceffantly by thefe Concuffions, 
muft be- put into a ‘vibrating Motion; and 
‘this “will be greater, as the Light is more, 
and weaker as it is lefs. 
This Vibration is fimple in the expanded 
Fibres ; ‘but it operates as varioufly as diftin- 
guifhably on thofe Clufters of them, which 
are placed at the Bafes of the main Rib, and 
of the feveral Footftalks of the Lobes. 
It is on the Operation of Light upon thefe 
interwoven Clufters’ of Fibres, that the Motion 
of the’ Leaves in gaining their different Po- 
fition depends ; ant ébhreguenttiy the Motion 
itfelf is various, according to the Conftruétion 
of thofe Clufters. 
The Effect of Light upon Bodies v we fee j is to 
put their Parts into a vibrating Motion; the 
Conftruction of pinnated Peivee is fuch as na- 
turally admits and propagates that Influence; 
and the Clufters of Fibres are ‘as a Kind ‘of 
Joints on which their Lobes are capable, un- 
der the Influence of Light, of a certain limited 
Motion. 
As the State of Water uninfluenced by Heat is 
5 and ‘that ‘they fup- Gabb. 
PA PONT Cg, 
0 aot” 
Ice, the natural Pofition of the Lobes in thefe - 
pinnated Leaves is drooping. This is their 
Pofture of Repofe : 
intended by the Author of Nature to remain; fot 
Vegetation is very imperfectly performed, ‘bile 
hay are in it. The Effect of Light is this. Vi- 
bration, and the Alteration of Potttion'4 in thofe 
Lobes. This is the Doétrine here’ advanced; and 
this is fupported by the following Experiments, 
I removed a Plant of the Abrus from a Stove, 
in the Evening of the feventh of Augift, and 
the Effect of moderate Day-light, without be- 
ing expoted to the immediate Action Of the Sun. 
This might be conceived the moft natural and 
equable Degree of Light; and therefore cael 
for the firft Experiments. | 
~The’ Lobes of the Leaves were nt Evening, 
when the Plant ‘was broug eht in, fallen perpen-. 
dicularly from the lade Rib, and clofed to-. 
gether by their under Sides. 
Thus they continued during the Nicht; in 
ina State of perfe&t Repofe. Half an Hour af- 
ter Day-break they began to feparaté; and ina 
ge irish of an Hour after eo rile flood hori-’ 
but in this they were not 
placed it in my Study, where it could have 
Me it ade hae Is Noo ay Be mene ie adie iar we ee os 
f : ; 7 ee x 
at ae 
