MOVEMENTS OF GROWING LEAVES AND FLORAL ORGANS. 873 
(upper) surface of the organ due to an increase, within certain limits, of the tempera- 
ture or of the intensity of light ; when the temperature and the intensity of light are 
diminishing the growth of the external surface is greater than that of the internal. 
In the former case the curvature is convex inwards (opening), in the latter it is 
convex outwards (closing ^). This is of course only the case when the diurnal condi- 
tion of the organ is the open one ; when the contrary is the case the meteoric 
influences affect the internal and external surfaces in just the opposite way. 
3. The curvature of the growing organs which are sensitive to meteoric in- 
fluences is not eff'ected like that of fully-developed motile organs by an alternating 
expansion and contraction of the tissue, but by an alternating more vigorous growth 
of one side and then of the other, so that the organ, whilst making these move- 
ments, continues to increase in length. This by no means excludes the possibility 
of a slight shortening of the concave side occurring temporarily just as in the case of 
growing tendrils and of geotropically curved stems and nodes of Grasses. 
4. Many of the organs now under consideration are especially sensitive to 
changes of temperature, others to variations in the intensity of light. Many 
are affected by very slight variations, others are less sensitive, and thus form a 
connection with those leaves and flowers which exhibit no such movements. In 
many cases each variation of temperature or of light has an immediate effect, in 
other cases the effect is produced only after the lapse of a considerable time since 
the last movement. 
5. From the various differences mentioned in the preceding paragraph it 
becomes evident why certain flowers (and leaves) open very early in the morning, 
and others only later in the day; and why it is that some are affected by every 
change of weather, whereas others complete their daily period with great exactitude. 
If we ask, finally, what the biological meaning of these phenomena may be 
in the economy of the plant, it is not easy at present to give any satisfactory answer 
in so far as leaves are concerned ^. The opening and closing of flowers, however, 
has an obvious connection with the process of pollination^; the flowers which are 
open by day are visited by the winged insects which effect pollination, and their 
closure in the evening, or during cold damp weather during the day, serves as 
a protection to the pollen in the anthers. Like many similar useful adaptations, 
these can be readily explained on the Darwinian theory, since they depend upon 
the further development of properties which belong also to allied plants but are less 
developed in them, and are not accompanied in them by corresponding collateral 
arrangements. 
^ In order to be able to apply the expressions ' opening ' and ' shutting ' to foliage-leaves these 
organs may be regarded as standing on a short axis or even as being in the bud. 
2 [Darwin includes these movements of leaves, as well as those described in the next chapter, 
under the head of tiyctitropic or sleep-movements. The object of the closing up of the leaves at 
night is, he believes, to diminish the radiation from them and thus to prevent injury due to an 
excessive fall of their temperature. In addition to the nightly sleep there is a diurnal sleep (Para- 
heliotropism of Darwin), in which the leaves present their margins to the incident light; the object 
of this is to protect their chlorophyll from the action of too intense light. (See Darwin, Move- 
ments of Plants, p. 445.— Wiesner, Die natürlichen Einrichtungen zum Schutze des Chlorophylls, 
Wien 1876. — Stahl, Ueb. sogennante Compasspflanzen, Jena 1881),] 
^ [On the protection of the pollen from the influence of the weather, see Kerner, Die Schutz- 
mittel des Pollens gegen die Nachtheile vorzeitiger Dislocation und Befeuchtung, Innsbrück 1873.] 
