Amphicarpaa monoica. 309 



a. Normal Position. — In diffuse daylight, in carl}- morn- 

 ing, or when shaded from the sun's rays, both simple- and 

 compound leaves assume a position which will here be 

 alluded to as "normal." The dorsiventral relation is 

 shown ; leaflets present a flat position, though occasionally 

 the terminal is slightly elevated. The angle of the petiole 

 with the main stem varies from 45 to 6o°. 



/>. Paraheliotropk Position. — If exposed to direct action from 

 the sun's rays or to a strong reflection, the lateral leaflets 

 turn the outer edges gradually upward, the upper surface 

 being inclined toward the terminal leaflet. As the intensity 

 of heat and light increases, the pulvini are stimulated more 

 and more, the angle assumed by the margin is 90° with a line 

 drawn through the surfaces in their normal position. The 

 terminal pulvinus rises about 45 °, thus elevating the leaflet. 

 The lateral pulvini rotate the leaflets at this stage; but 

 there is little change in the angle they make with the main 

 petiole. 



A further increase of the sun's action causes the lateral 

 leaflets to incline more toward the terminal, which rises usu- 

 ally 90 ; frequently the upper surfaces of the leaflets are in 

 direct contact, the under surfaces being outward. The 

 lateral pulvini have risen slightly ; but their angles are 

 difficult to estimate, as these blades have been so much 

 rotated. (Plate XXV, Fig. 1.) 



Often the terminal does not assume a perpendicular 

 position, but the apex points either right or left. It is 

 noticeable too, that the terminal leaflets on opposite sides 

 of the plant do not incline in the same direction, even if 

 the sun's rays are vertical. 



A series of questions which must yet remain unanswered 

 are suggested. Are these movements of advantage to the 

 plant ? Are the pulvini protected ? Is too rapid transpira- 

 tion retarded ? Are the movements due to heat, or to cer- 

 tain rays of light? 



For the present, a partial answer at least may be given 

 regarding the last. Plants were studied in the greenhouse 



