ACTION OF LEAVES. 51 



der this fact almost certain* It is, however, thought 

 by some, that leaves have no power of absorbing 

 water, even in an elastic state; and that the renova- 

 tion of plants by syringing is owing to a diminution 

 of perspiration. 



75. It is to the action of leaves, — to the decompo- 

 sition of their carbonic acid, and of their water; 

 to the separation of the aqueous particles of the sap 



* Mr. Knight entertained the opinion, that water is sometimes 

 absorbed by leaves to such an extent as to cause » descent of the 

 tap through the alburnum ;' a derangement of function to which he 

 even ascribed th£ attacks of mildew fungi upon plants. The secon- 

 dary and immediate causes, he says, of this disease, and of its con- 

 geners, " have long appeared to me to be the want of a sufficient 

 supply of moisture from the soil, with excess of humidity in the air, 

 particularly if the plants be exposed to a temperature below that 

 to which they have been accustomed. If damp fmd cold weather 

 in July succeed that which ha* been warm and bright, without the 

 intervention of sufficient rain to moisten the ground to some depth, 

 the wheat crop is generally much injured by mildew. I suspect 

 that in such cases an injurious absorption of moisture, by the leaves 

 and stems of the wheat plants, takes place : and I have proved that 

 under similar circumstances much water will be absorbed by the 

 leaves of trees, and carried downwards through their alburnous 

 substance ; though it is certainly through this substance that the 

 sap rises, under other circumstances. If a branch be taken from a 

 tree when its leaves are mature, and one leaf be kept constantly 

 wet, that, leaf will absorb moisture, and supply another leaf below 

 it upon the branch, even though all communication between them 

 through the bark be intersected ; and, if a similar absorption takes 

 place in the straws of wheat, or the stems of other plants, and 

 a retrograde motion of. the fluids be produced, I conceive that the 

 ascent of the true sap or organisable matter into the seed-vessels 

 must be retarded, and that it may become the food of the parasitical 

 plants, which then only may grow luxuriant and injurious."— 

 (Sort. Trana., i. 86.) 



