president’s address. 
597 
is then conveyed upwards through the alburnum or sapwood to the 
fibro-vascular bundles of the stalks and veins of the leaves. 
From there it is drawn off by a layer of cells surrounding the 
bundles, and is distributed throughout the leaf. On account of 
the continued pressure, the water transudes through the walls of 
the turgid cells into the intercellular spaces, and escapes from the 
stomata as watery vapour. In this way a current is set up, which 
is termed the transpiration current. The main use of this current 
is for keeping the leaves moist and cool, as otherwise on very hot 
days they would desiccate and die. On hot days, and especially if 
the air be dry, transpiration is very active ; whilst if the air is 
cool and contains moisture, very little transpiration goes on. But 
here a physiological factor comes into play. If the cells of the 
leaf were simple bladders the water would transude so completely 
that the cells would soon be emptied ; but their walls, as we have 
seen, are lined inside with living protoplasm, and this has a con- 
trolling effect over the amount of water that escapes ; anil besides, 
as soon as the guard cells of the stomata become flaccid, they 
approximate together and close the orifice. The leaves of plants are 
specially concerned with the interchange of gases, and play very 
little part in the absorption of water; even after a shower of 
rain very little, if any, water is taken in. The part where absorp- 
tion does take place is probably along the course of the veins. A 
flagging leaf may absorb just sufficient water to revive it, but not 
enough to render the cells turgid, even if the plant be in a moist 
atmosphere. A return to turgidity can only be brought about by 
the supply of water to the root. Many plants, however, such as 
mosses and lichens, freely absorb water by their surfaces, and 
recover very quickly after a little rain or dew, even though they 
had been perfectly desiccated. On a hot day the leaves of a 
succulent plant, such as a turnip or cabbage, will flag from exces- 
sive transpiration ; but as night comes on, the stomata close on 
account of the cold air, and the warm soil causes an abundance of 
water to be absorbed by the roots, the pressure of which is so 
great in the cells of the leaf that they are in danger of bursting. 
This, however, is prevented by the water transuding through the 
