PHYSIOLOGY 269 



The composite flowers of Taraxacum, Leontodon, and other Com- 

 posites, also the flowers of Nymphaea, Cacti, etc., open when illuminated, 

 and close when kept in darkness (Fig. 211). Variation of light 

 produces also unequal growth in foliage-leaves, particularly in those 

 of the Chenopodiaceae, Caryophyllaceae, and Balsaminaceae, and cause 

 them to assume so-called sleep positions. 



In many instances the movements of the floral leaves are produced by varia- 

 tions of light as well as of temperature ; for example, the flowers of the Tulip and 

 Crocus open in the light and close in the dark, although the temperature remains 

 constant. In the case of opposing external influences, the resulting direction of 

 the movement of the flower-leaves is determined by the one which is predominant. 

 The dependence of these movements upon different, and often opposing, influences, 

 together with the continuance of movements induced by previously operative 

 influences (after-effects, pp. 256, 272), was for a long time a difficult problem, and 

 obscured the discovery of their true cause. 



These movements, occasioned by variations in the illumination and 

 temperature, must not be confused with those of heliotropism and 

 thermotropism ; in both of which the movement induced in an organ 

 is dependent upon its relative position with respect to the source of 

 the light or heat, and not upon the varying intensity of the stimulus. 



3. Movements due to Changes of Turgor (Movements of Irritability) 



The various movements hitherto considered are, to a large extent, 

 the result of the action of forces acting on growth. These move- 

 ments were therefore confined to organs, or parts of organs, still in 

 a state of growth. In contrast to the almost universal immobility 

 of all fully-grown organs, it is particularly interesting to find that 

 some plants have found a means of carrying on vigorous movements 

 without the assistance of growth. 



It has already been shown (p. 166) that through the pressure of 

 increasing turgidity the elastic cell walls become greatly distended 

 and the cell cavity largely expanded, while, on the other hand, the cell 

 walls shrink and the cell becomes smaller when the turgor is 

 diminished (Fig. 167). It is to these changes in volume, which thus 

 result from alterations in turgor, that the varying movements of 

 fully-developed living organs are due. 



Such variation movements occur only in leaves (foliage and flower 

 leaves). These movements are especially noticeable in the compound 

 leaves of the Leguminosae and Oxalideae, and also in the leaflets of 

 Manilla (a water-fern). In the motile regions of these leaves special 

 masses of tissue are, both physiologically and anatomically, adapted 

 for the promotion of this form of movement. 



This tissue appears externally as a firm cushion or pulvinus, sharply distin- 

 guished from the rest of the leaf-stalk, and is the direct cause of the leaf move- 



