FOKOE OF SUCTION BY LEAVES. 69 



Since a plant does not perspire at night, and since its 

 absorbing points, the roots, remain during that period in con- 

 tact with the same humid medium as during the day, they will 

 attract fluid into the system of the plant during the night, and, 

 consequently, the weight of the individual will be increased, as 

 Hales found to be the case. In like manner, if plants iu the 

 shade are abundantly supplied with moisture at the roots, they 

 also will gain more than they can lose ; and, as this wiU be a 

 constant action, the result must necessarily be to render all 

 their parts soft and watery. , 



It is evident, from what has been stated, that leaves must 

 derive the food they digest from the earth through the medium 

 of the roots, and from the air ; and that they, while alive, 

 maintain a kind of perpetual sucking action upon the stem, 

 which is communicated to the spongelets. That this must be 

 of a very powerful nature is apparent from the fact, that the 

 smallest leaf at the extremity of the branch of a lofty tree must 

 assist in setting in action the absorbing power of roots, at a 

 distance equal, perhaps, to three thousand times its own length. 

 If this reciprocal action is not maintained without interrdption, 

 and if anything occurs to check it during the period of vege- 

 tation, the plant will suffer in proportion to the amount of 

 interruption. For example, if the roots are placed in a warmer 

 medium than the branches, and are thus induced to absorb fluid 

 faster than the slower action of the leaves can consume it, the 

 superfluous sap wiU burst through the stem and distend its 

 tissue till the excitability is impaired or destroyed. Or if, on 

 the other hand, a branch is caused to grow in a warm medium,- 

 while the roots remain in a very cold medium, the former 

 will consume the liquid sap faster than the latter can 

 supply it, and the consequence will be, that the leaves will 

 die, or the fruit will fall off, or the flowers be unable to set their 

 fruit, from want of a constant and sufficient supply of food ; or 

 the fruit will shrivel, or, as it is said, wiU " shank." Not that it 

 is necessary for the temperature of the earth and air to be 

 equal, for this does not happen in nature ; but it is requisite 

 that they should have some near relation to each other. 



It is generally, however, believed, that leaves absorb fluid 



