HELIOTROPISM 



463 



optimum for positive heliotropic curvature. Finally, by further increase in 

 light intensity, we reach a new liminal value, viz. that for negative curvature. 

 Both negative and positive curvature have for their object the placing of the 

 plant in the position of optimum light intensity. It may be possible to establish, 

 at least for some plant organs, that all light intensities which produce negative 

 curvature do not act equally, but that as the intensity increases the negative 

 curvature also at first increases and later decreases. We would thus have 

 a curve with three zeros, regarding the abscissae as indicating light intensities 

 and the ordinates as representing the amount of reaction (relatively their 

 reciprocals). Such a curve is illustrated at Fig. 146. 



As Oltmanns has pointed out (1892) the whole curve is not given by every 

 organism ; at the most we may expect that shade-loving plants (amongst which 

 we may include Phycomyces) will exhibit both positive and negative halves 

 of the curve. Photophilous plants, on the other hand, demand so much light 

 that we must be satisfied if we can prove generally that they are no longer 

 positively heliotropic when the light reaches a certain intensity, for we are 

 unable to recognize negative heliotropic curvature in them. On the other 

 hand, roots perhaps show only the negative section of the curve, for they 

 are negatively heliotropic only when the light is very bright, and to all lower 

 intensities they are quite indifferent. 



Iniensity uf liylit 



Fig. 146. Diagrammatic curve, to show the relation of heiiotropic- reaction to light intensity. 



The problem of heliotropic reaction is complicated by the fact that the 

 cardinal points of the curve (the zeros and maxima) are not constant in the case 

 of any selected plant. They vary both according to external and internal condi- 

 tions. Amongst the former light itself is by far the most important. Olt- 

 manns (1892) found that the longer Phycomyces was illuminated the more its 

 adaptability to light increased, for sporangiophores which were negatively 

 heliotropic at the commencement of the experiment became, after a few hours, 

 indifferent or even positively heliotropic. One culture, which had been strongly 

 illuminated for ten hours, showed next day a strong tendency to bend over 

 in the reverse direction. Further, the previous illumination has a great influence 

 on the initiation of the positive curvature, for it is known that the commence- 

 ment of the reaction takes place at a much lower intensity in etiolated plants 

 than in those which have been grown in light. Among internal factors which 

 influence light adaptability the age of the organ has to be considered. YoiDig 

 sporangiophores of Phycomyces are more adaptable than old ones ; indeed, the 

 latter respond more readily in negative manner. The heliotropic adaptability 

 of certain flowers has long been a familiar fact ; such flowers are positively 

 heliotropic when young but react negatively after being fertilized, e.g. 

 Linaria cymbalaria (Hofmeister, 1867) and many others (W'iesner, 1880 ; 

 Hansgirg, 1890). In these cases the power of adjustment is obviously of 

 service to the plant, for flowers exposed to light will be more readily seen 



