180 Macfarlane, The Sensitive Movements of some Flowering Plauts. 



ligbt intensity. My observations show that for any one species 

 of plant there is a definite Optimum that varies somewhat aceording 

 to temperature, moisture, etc. — and that up to the Optimum pomt 

 all of the rays contribute to maintain the diaheliotropic position. 

 Beyond this the more refrangible rays cease to be trophic, and 

 acting as a stimulant cause movements that correspond in amount 

 with the intensity of the light-stimulant. That at a definite tem- 

 perature the less refrangible and more refrangible rays are alike 

 trophic is highly probable, from the fact that plants grown for 

 days under all of the colored screens are not permanently injured, 

 even though in the active growing State. Thus the four plants of 

 Cassia Tora were under screen for seven days, and were then placed 

 in my plant house, where they have since grown well. It should 

 be said however that the growth of the plant from unde- the blue 

 screen, has been less active than in the case of those fromthe first 

 two. That the blue-violet as well as the red-yellow rays are 

 trophic, seems to be a probable explanation of the behavior of 

 plants when first placed under a red or orange-yellow screen, for 

 the closing of the leaflets then seems only to be accounted for by 

 the cutting off of the more refrangible rays of Optimum intensity, 

 and accompanying disturbance of the normal functions. The re- 

 expansion of the leaflets within a few hours at longest, proves that 

 the disturbance is temporary and not deep-seated. 



Oltmann's further says : „Alle dorsiventralen Organe eine 

 besondere Lage zum Licht einnehmen, indem sie demselben eine 

 ganz bestimmte Seite zukehren, welche ausserdem einen für jede 

 Intensität des Lichtes bestimmten Winkel mit den einfallenden 

 Strahlen bildet." 



The writer so far agrees with the above Statement, but be- 

 lieves that when the intensity of the more refrangible rays 

 reaches such a point that these rays cease to be trophic , the 

 highest limit of diaheliotropism is reached, and paraheliotropism 



ensues. 



It may be worth emphasizing here that sensitive movements 

 are most pronounced in tropical plants, are less so in sub-tropical 

 and warm-temperate species, and are rare or feebly expressed in 

 temperate and sub-artic plants. But, as is well known, leaveS' 

 that are exposed to an intense light show more rapid metabolic 

 changes than those that are shaded. Any change, therefore, in 

 the tissues of a plant which would insure protection of the lamina 

 from the intense blue-violet rays, and its exposure again advantage- 

 ously when these rays become subdued, would have every likli- 

 hood of perpetuation in sub-tropical and tropical regions, and 

 such is the state of matters as we find them. We do not know 

 accurately as yet the mechanism involved in a sensitive pulvinus, 

 or the changes effected on Stimulation of it, but anyone can 

 readily prove that every gradation from non sensitive to highly 

 sensitive leaves is met with in such groups as the Oxalideae and 



