510 
CURIOSITIES OF VEGETATION. 
The sensitive property residing in the 
leaves of these plants, fully entitles them to 
rank as vegetable curiosities. This property 
consists in the motion which the leaves dis- 
play on being touched or pressed ; a move- 
ment the nature of which has not heen yet 
properly explained. Like many other facts 
in natural history, this fact has heen ohserved, 
and watched, and commented on ; but the 
why and the wherefore have as yet escaped 
the penetration of men of science, and re- 
mains to this day a monument of the finite 
powers of the human mind. 
It is, however, interesting to examine some 
of the experiments and opinions recorded in 
connexion with this phenomenon. Duhamel 
ohserved that, at 8 a.m. of a fine September 
day, a leaf-stalk of a sensitive plant formed 
with the lower part of the stem an angle of 
135° ; upon being touched, it fell to an angle 
of 80° ; an hour afterwards it rose again to 
135°, and, on being touched a second time, 
fell again to 80° ; an hour and a half after- 
wards it rose to 145°, and, on being touched, 
fell to lo5°, where it remained till 5 p.m., 
when, upon being touched, it fell to 110°. In 
this case the susceptibility was greatest in the 
morning, or during the heat of the day. The 
plant does not always recover in the same 
way ; sometimes the common footstalk re- 
covers first, sometimes the lateral footstalks, 
and sometimes the leaflets. 
Henslow mentions, as one of the most 
striking means of eliciting the phenomenon, 
that of scorching a single leaflet in a candle, 
or by concentrating the sun's rays upon it by 
means of a lens. This leaflet, together with 
that opposite to it, will immediately move, in- 
clining forwards, and bringing their upper 
surfaces in contact. Other pairs nearest the 
one first stimulated will then close in succes- 
sion, in a similar manner, and at length the 
partial petioles will themselves fold together, 
by inclining upwards and forwards. Lastly, 
the influence is transmitted to the common 
petiole, which bends downwards, with its ex- 
tremity towards the ground ; a direction the 
reverse of that taken in the former cases. 
The effect is next continued to the nearest 
adjoining leaves, which fold up their leaflets 
and depress their petioles in a similar man- 
ner. If a plant is shaken roughly, all the 
leaflets close simultaneously, and the petioles 
droop together ; but if this agitation is long 
continued, the plant becomes accustomed to 
it, and after some lapse of time the leaflets 
again expand. 
The mechanism by which these movements 
are produced, resides in the thickened, or 
swollen bases of the petioles and leaflets. If 
ihe upper part of these swellings is cut 
away, the leaf remains erect, but if the lower 
part is removed it continues depressed ; hence 
it would appear that the elevation and de- 
pression of the leaf is owing to the elasticity 
of the tissue of which the swollen joint is 
composed ; and that the stimulus employed 
to produce motion tends to weaken the upper 
parts of these joints, in the case of the leaflets 
and partial petioles, but the lower part of 
those belonging to the main petioles — the 
contrary sides, continuing elastic, as before. 
How this effect is produced, or what is the 
law which regulates its action, it has been 
already stated, is not known. Whatever the 
causes, they are active from the early stages 
of the plant's existence, for the seed-lohes 
exhibit the property as soon as they have 
expanded. Dutrochet is of opinion, that the 
transmission of the stimulus from one leaf to 
another takes place along the stem, through 
the intervention of the ducts contained in the 
woody parts ; for he found, that, if both the 
pith and the bark were removed, the effect 
was not stopped ; whilst if the woody parts, 
which contain the ducts, were abstracted, it 
entirely ceased. 
It is not, says Miller, the light which causes 
the leaves to expand, as some have affirmed 
who have had no experience of these things ; 
for in tie longest days of summer they are 
generally contracted by five or six in the 
evening, when the sun remains above the 
horizon two or three hours longer ; and, 
although the glasses of the stove be covered 
close with shutters, to exclude, the light in the 
middle of the day, yet, if the air of the stove 
is warm, the leaves of the plants will continue 
fully expanded, as I have several times ob- 
served. Nor do these plants continue shut 
till the sun rises in the morning, for I have 
frequently found their leaves fully expanded 
by the break of day in the morning ; so that 
it is plain the light is not the cause of their 
expansion, nor the want of it that of their 
contraction. I have also observed that those 
plants which were placed in the greatest 
warmth in winter, continued vigorous, and 
retained the faculty of contracting on being 
touched, while those placed in a more moderate 
degree of warmth, had little or no motion." 
The roots of the Mimosa?, as of most, if 
not all the Acacias, an allied genus, have a 
very strong unpleasant odour, which has been 
compared to that of a common sewer. 
These are cultivable plants ; they may be 
regarded as tender annuals, though they are 
properly biennials. They are raised from 
seeds, which should be sown in a hot-bed, or 
hot-house, towards the end of February, in a 
compost in which light vegetable soil abounds. 
The seeds will speedily vegetate, and, when 
the first pair of true leaves are expanded, 
may be potted singly into small pots, in a 
