108 



FROST EFFECTS. 



[CH. IV 



with the mouth while the lamina is in water. Or the 

 air-pump may be applied. It is easy to ascertain the 

 pressure necessary for injection by the 

 following arrangement. 



The leaf is attached to one arm 

 of a T tube : by means of the second 

 arm suction is applied, and the third 

 ends in a bent tube dipping into 

 mercury. If the junction between 

 the stalk and the T tube is not quite 

 air-tight it is of no consequence, since 

 the leak affects the leaf and the mano- 

 meter equally. A good air-tight 

 junction may however be made by 

 Devaux's method 1 of melting the leaf 

 stalk into a funnel with gelatine G 

 as shown in fig. 21. FlG - 21 - Ex P- 12 - 



(121) Frost effects. 



The injection of the intercellular spaces with water 

 can be observed on the frozen leaves of certain evergreens. 

 In a hard frost the leaves of the ivy (Hedera) have a 

 semi-transparent, dark green appearance, like, but not so 

 dark as, the colour of a water-logged leaf. If the leaf is 

 pinched between the finger and thumb the normal light 

 green colour returns to the under-surface : the same effect 

 may be produced by dipping a corner of the leaf in 

 lukewarm water. If however the whole leaf including the 

 cut stalk is thawed under water, it does not become light 



1 Ann. Sc. Nat. 1889. 



