348 BOTANY 



PART I 



during the whole reaction-time. A much shorter period of stimulation 

 is sufficient to obtain a geotropic curvature as an after effect from the 

 organ which has been replaced in the vertical position. The minimal 

 period of stimulation after which a visible curvature results is termed 

 the PRESENTATION-TIME. The law of amount of stimulus only applies 

 to stimuli which last as long or somewhat longer than the presentation- 

 time ; the presentation-time is thus inversely proportional to the 

 intensity of the stimulus. No corresponding increase of geotropic 

 curvature follows larger amounts of stimulus. 



Stimuli below the presentation time are not without effect. On 

 repetition they are summed up and result in a curvature when the 

 sum of separate stimuli amounts to the presentation-time, if the 

 intervals between the separate stimuli have not been too great. A 

 lower limit for the duration of separate stimuli has not as yet been 

 determined. 



The law of amount of stimulus also applies when the centrifugal 

 force or gravity acts obliquely on an orthotropous part of a plant. 

 The effect of gravity diminishes in proportion to the sine of the angle 

 of incidence; if at 90 it = 1, it will be = 0'5 at 30. Thus only the 

 pressure at right angles to the long axis is effective. 



2. PHOTOTROPISM (HELIOTROPISM) ( 105 ) 



A good opportunity for the observation of heliotropic phenomena 

 is afforded by ordinary window-plants. The stems of such plants do 

 not grow erect as in the open, but are inclined towards the window, 

 and the leaves are all turned towards the light. The leaf- stalks and 

 stems are accordingly ORTHOTROPIC and POSITIVELY PHOTOTROPIC. 

 In contrast to these organs the leaf -blades take up a position at 

 right angles to the rays of light in order to receive as much light as 

 possible. They are DIAPHOTOTROPIC, or TRANSVERSELY HELIOTROPIC, 

 in the strictest sense. If among the plants there should be one 

 with aerial roots, Chlorophytum for instance, an example of NEGATIVE 

 PHOTOTROPISM will be afforded, as the aerial roots will be found to 

 grow away from the window and turn towards the room. In Fig. 

 283 the phototropic curvatures which take place in a water culture 

 of a seedling of the White Mustard are represented. 



Sensibility to phototropic influences is prevalent throughout the vegetable 

 kingdom. Even organs like many roots, which are never under ordinary 

 circumstances exposed to the light, often exhibit phototropic irritability. Positive 

 phototropism is the rule with aerial vegetative axes. Negative phototropism is 

 much less frequent ; it is observed in aerial roots, and sometimes also in climbing 

 roots (Ficus stipulate*,, Begonia scandens), in the hypocotyl of germinating 

 Mistletoe, in many, but not all, earth roots (Sinapis, Helianthus), in tendrils 

 (chiefly in those with attaching discs), and in the stems of some climbers. By 

 means of their negative heliotropic character, the organs for climbing and attach- 



