126 HOUSE, GARDEN, AND FIELD 



have executed as usual the movement by which they are 

 accustomed to protect the leaf from the cold of night. 



A clover leaf generally erects itself before the leaflets 

 close ; the leaf and leaf-stalk are thus gathered beneath 

 the radiating surface, which, as we have seen, is reduced 

 to a minimum. 



Clover as a rule opens by day and closes by night. 

 Artificial darkness may cause it to fold up its leaves, but 

 not always. Now and then we are surprised to find that 

 clover which we had locked up in a cupboard overnight 

 has its leaves wide open when we come to look at it next 

 morning. The plant gets into what we must call a habit 

 of opening at a fixed hour, and goes on doing it even in 

 the dark. The habit is soon lost, if the hours of light 

 and dark are changed. 1 



On a still, bright summer day note the earliest hour at 

 which clover leaves begin to close. If it were not so 

 troublesome to watch plants in the early morning, I would 

 ask you to determine also the hour at which they begin to 

 open. You would find that the leaflets are hardly ever 

 stationary. When they are fully expanded, they will soon 

 begin to close, and when they are completely closed, they 

 will soon begin to open. This is not true of all plants 

 whose leaves take a sleep-position. 



In the next lesson clover is compared with wood sorrel. 



XXVI. WOOD SORREL. 



The leaves of wood sorrel have almost the same shape 

 as those of clover, and they can open and close, but in a 

 way of their own. In closing, the leaf -stalk generally 

 erects itself ; then each of the three leaflets droops until 

 it hangs nearly vertical (Fig. 33) ; lastly, it bends inwards 

 along its midrib until its under surface fits close against 

 those of the other two. Why this difference ? Why 



1 F. Darwin and D. F. M. Pertz, Annals of Botany, vol. vi. p. 245, 

 and vol. xvii. p. 93. 



