THE MOVEMENTS OF PLANT OKOANS. 
97 
actin<^ afresh they came out, and by repeating the process tlie root 
" threaded " the gau/e. 
Another important property of the growing point is the extreme 
sensitiveness to mechanical obstructions which induces the apex to turn 
<iway from them. Mr. Darwin observes that this is the only known instance 
of an organ turning aiuay from an object. By fixing cards to one side of 
the tips of vertically growing radicles they were caused to turn upwards, as if 
trying to escape from the cards, sometimes even forming complete circles ; 
and in one case a radicle actually tied itself into a knot. This sensitive- 
ness is confined to the tips for a length of from 1 mm. to 1-5 mm., while 
the upper adjoining part of the radicle, for a length of from 0 or 7 to 
even 12 nnn., is excited to bend away from the side which has been 
irritated. 
Another feature of roots is to grow towards the darker side. This is 
well seen in the aerial roots of Ivy against a wall, or of Orchids, so that 
they can cling to the supporting objects. Creeping stems have the 
same tendency to avoid light ; hence they keep along the ground instead 
of rising vertically. 
Stems. — Stems which grow^ erect support themselves by putting on 
sufficient mechanical tissues — wood-fibres, liber-fibres, sclerenchymatous 
cells and collenchyma — in order to resist the influence of gravity to make 
them fall down. Circumnutation succeeds grow^th, and becomes a 
valuable acquisition in the case of climbing stems or twiners, such as 
the Convolvulus, Hop, &c. 
Expansion of Leaf-buds. — Asyoungleaves escape from buds in spring- 
time, the petioles and blades undergo movements much resembling those 
involved in the sleep of plants, and for the same object. In the immature 
state the commonest condition of vernation is to have the two halves of 
a blade folded together or " conduplicate," the upper surfaces being in 
contact. The petiole at once curves downwards. This places the 
blade in a vertical position. Thus the two objects are gained — viz. of 
protecting the upper surfaces especially, and the avoiding having any 
surface horizontal. Thereby radiation, and any consequent injury by 
chill, is greatly avoided. Mr. Darwin found that these objects were aiso 
secured by the process of sleep or " nyctitropic " movements. 
Taking Clover as an example, a young leaf has each leaflet con- 
duplicate : all three are pressed together, and stand in a vertical plane. 
When, how^ever, an adult leaf sleeps, the three leaflets having been 
horizontal by day, they do not resume the above condition ; but the two 
basal leaflets first rotate and stand vertically ; they then approach one 
another until their upper surfaces are in contact. Lastly, the terminal 
leaflet passes through nearly 180° and comes down upon the edges of the 
others. It then partially closes like a sloping roof over them, its upper 
surface becoming now the underside. The various methods of sleep 
and the movements connected with them in dift'erent plants can be 
studied in Darwin's work.* 
Flowers. — There are many instances of the various parts of flow^ers 
moving under the influence of stimuli : they are mostly connected with the 
process of pollination. Commencing with bracts, it may be easily seen how 
^- The Power of Movement in Plants. 
