. 
Oétob. 
Manner in the F ootitalk ; with delicate Clufters 
at its Bafe: and they are originally fent from 
the Pith within the Stem. | 
- Thefe Fibres ferve to fupport the Leaf in its 
proper Pofition ; and therefore whatever external 
or internal Caufe affeéts them, will change that 
‘Pofition. 
This is the Conftru€tion of the Part to be 
influenced ; the Queftion remains, what it is 
that affects it; and to know that we are to 
examine whatever may have fuch Power. 
_ Leaves thus conftruécted are always furround- 
ed by the Air; and they are occafionally 
and varioufly influenced by Heat, Light, and 
- Moifture: the Air alfo itfelf being in a con- 
tinual State of Variation, its Alterations are 
to be confidered as poffible fubordinate Caufes 
of Change. A 
Thefe Things, and thefe only, come within 
Contact of Plants, or within the Sphere of In- 
fluence. Bodies do not affe& Bodies, but on 
Contact, or within that Sphere: therefore’ the 
Caufe of the Change of Pofition in Leaves, 
is to be fought among thefe Agents, and no 
other. he” a, 
They are naturally complicated, and they a@ 
on moft Occafions together, We are there- 
fore to obferve, firft, what Effects refult from 
their mutual Combinations in a State of Na- 
ture: and having affigned in thefe Cafes the 
Effect to the proper and particular Caufe, from 
this Power of that Agent, whichfoever it is, 
that acts thus in Concert with the reft, we 
~ may deduce its Operations fingly. 
Pinnated Leaves, fuch as are compofed of 
numerous Lobes, or fmaller Leaves placed on 
a common Foot-ftalk, are moft remarkable for 
their Change of Pofition : it will be therefore 
beft to wave all other Confiderations here; and 
obferve folely the Condition of thefe. 
The four Agents we have named are univer- 
fal; but we fhall find their Operation differs in 
various Climates. In thefe temperate Kingdoms, 
our native Plants which have pinnated Leaves, 
naturally hold the Lobes nearly horizontal, and 
fhew but a moderate Senfibility in this Re- 
fpect: in the hotter Regions, the ufual Pofition 
of the Lobes in thefe Plants is turning up- 
wards, and they are extremely fufceptible of 
Change of Pofture; in Agypt moft of all: in 
the more Northern Nations, on the contrary, 
they fcarce ever reach an horizontal Pofition, 
and they fhew very little Change on any Oc- 
cafion. — 
As we fee different Appearances in thefe 
Parts of Plants in hot, temperate, and cold 
Climates; Obfervations of a like Kind fhew, 
_they are not lefs varioufly affected in the fame 
Kingdom in rainy and fair Seafons. In thofe 
Places where there are regular Periods of rainy 
Weather, the Change in the Face of the pinna- 
ted Plants is very great, and certain : thofe 
which in the fair Months carry their Lobes 
in an obtufe Angle upwards, conftantly hang 
them obtufely downward in the Time of the Rains. 
3 
A COMPLEAT BODY 
_ Effect moft of all; 
Thefe are the Obfervations of curious Voy- O@ob. 
agers; and they have been confirmed by the 
| immediate Notice of Botanifts in thofe Places. 
The firft would feem at once to give the Effect to 
Heat; and the other to Moifture: but farther 
Obfervation fhews it is otherwife. Linn vs 
has juftly obferved, that the fame Thing happens 
to Plants in the Stove, where there is no Alte- 
ration in Point of Heat; and I have found by 
careful Tryal, that Moifture has in the fame Re... 
{pect no Effect. I have for this Purpofe watered 
fome Plants almoft to Deftruction, and left others 
of the fame Kind dry; and no Alteration has 
been made by this: they all. expand, or raife 
their Leaves in the Morning, and drop them 
in the Evening; at the fame Hour and in 
the fame Degree. 
Two of the four natural Agents, Heat and 
Moifture, are therefore excluded from any Share 
in this Effect: the Air is too univerfal, and its 
‘Changes too much depend on thefe to be 
admitted in the Refearch. "The Attention there- 
fore falls on Light alone: and I have found 
by many Experiments, that the Change of Po- 
fition in the Leaves of Plants at different Pe- 
riods of the Day and Night, is owing to this A- 
gent and no other. : 
| produce the Effect; but that Light inevitably 
muft. 
| Thefe are the Difcoveries on which the ever- 
lafting Seal of Truth is {tamped; which Reafon 
dictates; and which Experiments confirm, 
We have propofed to fearch the latent Principle 
of this Change in the Qualities of Bodies, and 
their natural Operations upon one- another, 
proper to fix on, 
: let us chufe for this Pur- | 
pofe an Apyptian Plant, fince thofe fhew the 
and among thefe none can be 
more proper than the rus, celebrated for it by 
the earlier Writers. 
The Leaf of the rus coniifts of thirteen Pairs 
of Lobes, fixed by very fhort and extreamly 
flender Footftalks to the middle Rib; and this — 
to the main Stem of the Plant. 
_ Examining its internal Structure by the Mi- 
crofcope, we find a Number of delicate Fibres, 
rifing from the central Part of the main Stem, 
and continued in a Courfe obliquely upwards, 
through the intermediate Parts, and to the 
Outfide of the Rind. Here they fwell; and 
run into feveral regular Clufters, fpreading 
downward and on each Side: and thefe form 
(under the continued Covering of the Stem) the 
Bafe of the common F ootitalk, or middle Rib of 
the Leaf, 
From 
