6 9 



absorbed fluids. It has been found that the careful removal 

 of the heart-wood of trees does not prevent the supply of liquid 

 to the branches from the roots ; but if the layers of sap-wood 

 are removed, the upper parts of the tree die from desiccation 

 even when the bark is left uninjured, except to such an extent 

 as is sufficient to allow of removing the wood beneath. What 

 advantage does the plant gain then by the exhalation of such a 

 large quantity of water as this ? for in the Sunflower {Helian- 

 thus annuus) on a summer day, the weight of the water 

 exhaled is equal to the weight of the plant itself. The answer 

 is that as fast as water is being transpired by the leaves a de- 

 mand for more is set up, and the plant takes up as much 

 fluid as it is possible for it to do by the roots, but sometimes 

 it cannot get enough. It takes up, not water alone, but 

 water containing salts in solution, which are then present in 

 comparatively large quantities in the plant for its nutrition. 

 Transpiration is then the force by which the plant gains a 

 supply of energy for its growth. It is greatest when the ac- 

 tivity of the plant is greatest ; in the Sunflower, when the 

 leaves are largest, and the temperature high, and the light 

 intense, the stomata are most widely open, and it is just then 

 that the largest amount of salts are needed for nutrition. The 

 transpiration of a plant does not bear any direct relation to the 

 amount of moisture absorbed by its roots, for under certain cir- 

 cumstances, as on particularly hot days, when the atmosphere is 

 flashing with a bright light, we get a condition where the amount 

 of transpiration is in excess of the amount of absorption, and what 

 is known as withering or wilting of the plant occurs ; the leaves 

 of the tree or herbaceous plant giving out more water than is 

 taken in by the roots, so that the cells remain no longer tense 

 but become flaccid, the plant droops, and if this condition be 

 long continued the plant may die. If the activity of the root 

 absorption be in any way impaired and diminished, as, e. g. t if 

 the root be too much cooled, the plant withers. Again, a plant 

 when newly transplanted droops for a time, because the roots, 

 though they are supplied adequately with water, are incapable 

 of taking up the requisite amount of water fast enough until 



