54 ON PROPAGATING TREES. 



organic solid form ; and the cutting, in consequence, necessarily 

 becomes, to some extent, exhausted. 



Summer cuttings possess the advantage of having mature" and 

 efficient foliage, but such foliage is easily injured or destroyed, 

 and if it be not carefully and skilfully managed, it dies. These 

 cuttings, such as I have usually seen employed, have some mature 

 and efficient foliage, and other foliage which is young and grow- 

 ing, and, consequently, two distinct processes are going on at the 

 same time within them, which operate in opposition to each other. 

 By the mature leaves, carbon, under the influence of light, is 

 taken up from the surrounding atmosphere, and arterial sap is 

 generated. The young and immature leaves, on the contrary, 

 vitiate the air in which they grow by throwing off Carbon ; and 

 they expend, in adding to their own bulk that which ought to be 

 expended in the creation of shoots. This circumstance respect- 

 ing the different operations of immature and mature leaves, upon 

 the surrounding air, presented itself to the early labourers in 

 pnenmatic chemistry. Dr. Priestley noticed the discharge of 

 Oxygen'gas, or dephligisticated air (as it was then called) from 

 mature leaves. Scheele, making, as he supposed, a similar ex- 

 periment upon the young leaves of germinating beans, found these 

 to vitiate air in which they grew. These results were then sup- 

 posed to be widely at variance with each other, but subsequent 

 experience has proved both philosophers to have been equally 



correct. 



I possess many seedling young trees of the Ulmus campestris, 

 or Suberosa, or Glabra, for the widely varying characters of my 

 seedling trees, satisfy me, that these three supposed species are 

 varieties only of a single species One of these seedling plants 

 presented a form of growth which induced me to wish to propa- 

 gate from it. It shows a strong disposition to aspire to a very 

 o-reat height with a single straight stem, and with only very small 

 lateral branches, and to be therefore, calculated to afford sound 

 timber of great length and bulk, which is peculiarly valuable, 

 and difficult to be obtained, for the keels of large ships; and the 

 original tree is growing with very great rapidity in a poor soil 

 and cold climate. 



The stem of this tree, near the ground, presented, in July, 

 many very slender shoots, about three inches long. These were 

 then pulled off and reduced to about an inch in length, with a 

 sinole mature leaf upon the upper end of each, and the cuttings 



