DIV. ii PHYSIOLOGY 351 



In very bright light the transverse position of the leaves may become changed to 

 a position more or less in a line with the direction of the more intense light rays. 

 In assuming a more perpendicular position to avoid the direct rays of the midday 

 sun, the leaf-blades of Lactuca Scariola and the North American Silphium laci- 

 niatum and the leaf- like shoots of some Cacti take the direction of north and south, 

 and so are often referred to as COMPASS PLANTS. The foliage leaf has thus, like 

 the chloroplast of Mesocarpus, the power of assuming either a profile or a full- 

 face position, and thus regulating the amount of light received. 



A number of foliage leaves possess pulvini (Fig. 132) at the base of 

 the petiole, and also at the bases of secondary and tertiary branchings ; 

 variation movements are effected by the aid of these. In this way 

 these leaves are able to change their position throughout life, and at 

 any moment to assume the position which affords them the optimal 

 supply of light. " *rhey do not have a fixed light-position determined 

 by the strongest illumination during their development, but they 

 sometimes expose their edges and sometimes their surface to the light. 



ALTERATION OF TONE ( 10T ). A particular part of a plant does not 

 react always in the same way to one and the same stimulus ; the mode 

 of reaction may be altered by age or other influences. In this sense 

 the terms " tone " and " change of tone " are used. 



The flower-stalks of Linaria cymbalaria are at first positively phototropic. 

 After pollination, however, they become negatively phototropic, and as they 

 elongate they push their fruits into the crevices of the walls and rocks on which the 

 plant grows (p. 281). 



Among external factors that alter the tone the amount of illumination itself is 

 particularly important. Small amounts of light falling from one side on Avena 

 produce without exception a positive phototropic curvature ; larger amounts give 

 a weaker positive soon followed by a negative curvature ; still larger amounts give 

 a purely negative reaction. With further increase in the illumination a positive 

 reaction is again obtained, and later a weakened positive if not a negative reaction. 

 How far the intensity of the illumination also influences the results cannot be 

 discussed here. 



Phototropism, like geotropism, is a PHENOMENON OF IRRITABILITY ( 108 ). 

 In it the perception, conduction, and reaction of the stimulus can also 

 be distinguished ; there are also presentation-time and reaction-time. 

 Further, the law of amount of stimulus holds, and separate stimuli 

 which are individually ineffective can be added together to produce a 

 reaction. 



Localisation of Phototropic Perception. Often the stimulus of light is received 

 at the same place that the movement is effected. In certain leaves, however, the 

 lamina is able to perceive a phototropic stimulus without being able to carry out 

 the corresponding movement ; this takes place only after the stimulus has been 

 conducted to the leaf-stalk. It is true that the leaf-stalk can also react to direct 

 stimulation, but as a rule the dominant impulse proceeds from the lamina. Still 

 more striking relations are met with in the seedlings of certain Grasses ; in some 

 Paniceae only the tip of the so-called cotyledon can be phototropically stimulated, 

 and only the hypocotyledonary segment of the stem, separated by some distance 



