EXUDATION-PRESSURE : ROOT-PRESSURE 189 



and stem is not sufiScient to force water to the top of tall trees, 

 it brings about the introduction of water into the conducting 

 channels and helps in the rapid translocation of water throughout 

 the vascular tissues of the plant. 



When the absorptive activity of the root of a plant is encouraged 

 by warmth of the soil and at the same time the loss of water in 

 the form of vapour from the leaves is diminished or prevented 

 by a damp atmosphere, the plant becomes overcharged and 

 water is forced out of the tips and edges of the leaves in drops 

 which are frequently mistaken for dew-drops. 



This emission of drops of water may be often observed on the 

 tips and edges of the leaves of such plants as balsams, 'Arum 

 lilies ' and fuchsias when growing in warm houses in which a 

 damp atmosphere is maintained. Similar drops are sometimes 



nseen in the early morning on the tips and edges of 

 the leaves of species of Tropceohim, Alchemilla, 

 and many wild plants after a warm night when the 

 7 sky has been overcast. 



The 'bleeding' of cut stems and the exudation 

 of drops of water from uncut plants is not caused 

 exclusively by the osmotic pressure of the cells 

 in the root, but is due in some degree to the 

 parenchymatous cells of the leaves and the 

 medullary rays and wood parenchyma of the stem, 

 for ' bleeding ' from the cut end of a leafy stem 

 which has no connection with a root can often 

 be induced by immersing its young and easily- 

 wetted leaves and stem completely in water. 



The osmotic pressure which results in the 

 'bleeding' of plants when cut, or the forcible 

 emission of drops of water from leaves and other 

 parts is a general phenomenon observable in 

 Fig. 84, greater or lesser degree throughout the body of 



the plant ; it is best termed ' exudation-pressure ' or ' bleeding- 



