CHAPTER IV. MOVEMENTS OF PLANTS. 



2 53 



Fig. 465. 



Fig. 466. 



(6) Nyciitropic, or Sleep Movements. The leaves of Oxahs, 

 of Clover, of the Acacias, and the compound leaves of many 

 other plants, have been observed to assume positions quite 

 different at night from those they occupy during the day. See 

 Figs. 465 and 466. By day the leaflets are expanded so as to 

 expose as large a surface as possible to the light, but at night 

 they droop and become pendant from the axis on which they are 



borne, or, in some in- 

 stances, fold together 

 so as to present to the 

 sky as little surface as 

 possible. These move- 

 ments are serviceable 

 to the plant in prevent- 

 ing excessive radiation 

 of heat at night. In 

 many plants the com- 

 bined upper surfaces 

 of the leaves amount to 

 an enormous total, and 

 if this be spread out 

 to the sky, on a clear 

 night, the loss of heat by radiation must be very great — so great, 

 in fact, that serious injury to delicate tissues would often result, 

 especially in the case of plants inhabiting dry regions, or open 

 plains. Experiment has proved that when the leaves of nycti- 

 topic plants are pinned out horizontally, so that they cannot 

 close, they do suffer injury from this cause, the leaflets often 

 turning brown and dying after a night's exposure. 



The cotyledons of many germinating seeds of dicotyledonous 

 plants exhibit nyctitropic movements in a conspicuous manner. 

 Sometimes they droop at night like the leaflets of Oxalis, while 

 in other cases they rise from the horizontal to the vertical posi- 

 tion, closing over the plumule, and thus protecting it, as well as 

 their own upper surfaces. 



The flowers of many plants also show similar movements. 

 Some open in sunshine and close at night, or in cloudy weather ; 

 others, like the Evening Primrose and the White Lychnis, have 

 the opposite habit of opening by night and closing by day. 



Fig. 465. — Leaf of White Clover with the leaflets in 

 their diurnal position. 



Fig. 466. — Leaf of White Clover with leaflets in the 

 position which they assume at night. 



