324 LIFE : OUTLINES OF GENERAL BIOLOGY 



take account of the internal structure and activities of the parts 

 concerned, and of such puzzles as the reversal of the phototropism 

 in certain circumstances. Thus the flower-stalk which spreads the 

 flowers of the toad-flax {Linaria Cymbalaria) to the sun, may at a 

 later stage become negatively phototropic and push the fruit into a 

 crevice in the wall. 



Opening and Closing of Flowers. — While the growth-move- 

 ments of a shoot bend it in turn to all points of the compass, the 

 movements of leaves and floral parts are in one plane. The leaves 

 rise and sink; the flowers, or it may be inflorescences, open and 

 close. As long as there is some growth in the leaf or its stalk, in 

 sepal, petal, or bract, these so-called "nastic" curvatures occur. 

 The crocus flower opens in warmth and closes in cold; the dandelion 

 inflorescence opens in sunshine and closes on a cloudy day. So 

 definitely sensitive are some flowers to the external changes of 

 illumination and temperature that attempts have been made to 

 arrange a "floral clock" with a succession of plants opening at 

 successive hours; but the clock has, of course, to be consulted good- 

 humouredly. All the movements of this "nastic" type depend on 

 the unequal growth of the two surfaces of the organ. Thus, under 

 increased warmth the inner faces of the parts of the tulip perianth 

 grow more rapidly than the outer faces, and so the flower opens. 



Motor Organs. — In some cases there are special motor orgams at 

 strategic points, and these effect the movement of parts by rapid 

 changes in the turgidity of some of their component cells. Thus at 

 the bases of the leaflets in the clovers and wood-sorrels there are 

 specialised cushions, which fold the leaflets together into a "sleeping" 

 position, and raise them again when the conditions change. Similarly 

 at the bases of the pinnules of the Sensitive Plant, at the base of 

 the four divisions of the leaf, and at the base of the main leaf-stalk, 

 then* are motor "cushions". The stamens of the barberry, which 

 move when touched by an insect-visitor's feet (or in experimental 

 conditions by a bristle), have a basal motor organ. Another familiar 

 instance is the bilobcd stigma of the musk (Mimulus), which closes 

 on pollen grains or when artificially touched with the tip of a leaf 

 of grass. 



In a typical motor organ at the base of a leaflet, there is a rind 

 of thin-walled sappy cells, and the fibro-vascular bundles lie close 

 together near the centre. When the turgor or hydrostatic pressure of 

 the cells on one side or surface of the cushion is lessened, that side 

 or surface becomes concave, and this brings about a movement of 

 the distal part. A recovery from the flaccid state of the cells reverses 

 the movement; and all the plant's movements brought about by 

 changed permeability and turgor of cells are similarly reversible. 



The stimuli which induce the turgor movements may be changes 

 in illumination and temperature, as in the badly named "sleep" 



