MEASURES FOR PROTECTING GROWING PLANTS FROM LOSS OK HEAT. 



531 



substwnces from inor<;;uiic food can bo carrieil on iu tlieiu. These cotyledons are 

 frequently seen to increase in extent, and to grow and function exactlj' like foliage- 

 loaves. It would certainlj' be a great disjidvantage to green cotyledons of this 

 kind if they were obliged to give up either partially or perhaps entirely in the 

 following night the heat received during the day. In neighbourhoods where the 

 greater part of the seeds germinate at a low temperature, at the close of winter 

 at a time when the nights are still long, warmth must be economized as far as 

 pnvcticable, and especially must the loss of heat from the cotyleilons hy nocturnal 



Fig. 132.— Periodic l)eniling of Flowers and Inflorescences. 



I The umbel of the Carrot, day position. 2 The same umbel, night position. » Flower of Pansy, (ia.v position. 



* The same flower, night position. 



radiation be prevented. This is accomplished by the cotyledons, which are 

 separated from one another like the leaves of a book, and whose broad surfaces are 

 turned towards the sky, closing together when the sun sets, and again assuming 

 that position which they occupied in the quiescent seed. In this way the two 

 cotyledons now have their narrow edges turned skywards, and the loss of heat 

 in the night is as much as possible minimized. This movement of the cotyledons, 

 which on cloudless evenings and in exposed spaces occurs more quickly than under 

 cloudy skies and in places which are overshaded by trees, has also the advantage 

 that the small foliage -leaves, which are visible on the rudiments of the shoot 

 between the cotyledons, are covered over through the night. When the morning 

 breaks, and the danger of excessive loss of heat is passed, the cotyledons again open 

 out in order to sun themselves afresh to their full. 



