76 THE TISSUES OF PLANTS 



nutrition is carried on in both animals and plants, proceeds 

 during the period of repose. The heart continues its pul- 

 sations, and the blood is aerated in the lungs. In the 

 Hedysarum gyrans, we have a parallel instance of the same 

 perpetuity of motion. 



Fig. 20. 



Compound leaf of Hedysarum gyrans, having two pairs of leaflets, 5, articu- 

 lated to the petiole, and a large terminal leaflet. 



The terminal leaflet appears to be peculiarly sensitive to 

 light. It takes the position represented in the figure 

 during the night, but becomes horizontal during the day- 

 time, its midrib forming, with the petiole, a continuous 

 and direct line. The terminal leaf is, however, manifestly 

 depressed if the plant is only placed in the shade for a few 

 minutes. In the changes of its position with reference to 

 the ever varying intensity of light throughout the day, the 

 terminal leaf forms, in fact, a natural photometer or light 

 measurer of great delicacy and power. 



The movements of the lateral leaflets are, on the con- 

 trary, entirely independent of the influences of light, and 



