222 
sap previously in the cutting assumes an 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 growing; 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 respecting the 
different operations of immature and mature leaves upon the 
surrounding air presented itself to the early labourers in pneu- 
matic chemistry. Dr. Priestly noticed the discharge of oxygen 
gas, or dephlogisticated air (as it was then called), from mature 
leaves; Scheele making, as he supposed, a similar experiment 
upon the young leaves of germinating beans, found these to 
vitiate air in which tliey 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 young seedling 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 pro- 
pagate from it. It shows a strong disposition to aspire to a 
very great 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 pecu- 
liarly 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 
