CIECTJLATION- OF SAP. 93 



the cwlls, but in the operation a kind of selection or choice 

 is made ol materials passing through them ; but this 

 elective power appears only to extend to those elements 

 ■which are of a congenial nature to the whole or some 

 part of the plant, for when certain poisons are presented 

 to the roots or leaves they are unable to resist them. 

 This act of non-resistance does not prove that plants have 

 bo power of selection of nutrients, but merely that they 

 are incapable of resisting certain elements which are 

 inimical to their health and growth. The difference in 

 the density of liquid poisons presented to the roots may 

 also have some influence upon the elective powers ; and 

 it is well known that some mineral poisons, when much 

 diluted, will be absorbed by plants without any apparent 

 injury, while stronger solutions will cause death, as they 

 also do when given to animals in large doses. Certain 

 gases are also highly injurious to plants, being readily 

 absorbed by both roots and leaves. Sulphuretted hydro- 

 gen gas, says Dr. Balfour, attacks the leaves at the tips 

 first, gradually extending to the leaf-stalk, which would 

 seem to show that it followed the return flow of sap. 

 Sulphurous acid gas is highly injurious to plants, as 

 many a gardener has learned to his cost through the 

 accidental ignition of sulphur in houses filled with plants. 

 But the gas which most interests the practical cultivator 

 of planbs is the one known as carbonic acid gas, for it is 

 from this that the carbon, the most abundant single 

 element of plants, is obtained. Carbon also makes up 

 some forty to fifty per cent, of the bulk of the ordinary 

 plants cultivated for food, and in trees the proportion is 

 still greater, all of which is supposed to be derived from 

 carbonic acid gas, but just how it is obtained or fixed in 

 the form of plant cells has been a mooted question among 

 vegetable physiologists and chemists ever since they began 

 to investigate the principles of growth and composition 

 of plants. This carbon ghost will neither remain passive 



