90 CIRCULATION OP SAP. 



at their base a bud from which in like manner addi- 

 tional leaves make their appearance. The functions 

 of the little plant are now as complete as they are in 

 the monster tree. The delicate stem is supplied with 

 its capillary tubes, which carry the nourishment from 

 the root, and distribute it throughout the different 

 parts of the plant. These vessels 

 perform the same part to the 

 tree as the veins and arteries 

 to the human body. One set, 

 comparable to the veins, carry 

 the sap through the trunk and 

 branches to the leaves, where 

 it is spread out in the minute 

 reticulated net-work on their 

 upper surfaces, and like the 

 Post-Oak. blood in the lungs is there ex- 



posed to the action of the light and air, absorbing 

 from the latter a portion of carbon, and at the same 

 time parting with its oxygen. This process purifies 

 the sap, and renders it fit for the nourishment of the 

 tree, when a new set of vessels, similar in their omce 

 to the arteries, distribute it to the different parts 

 where it is required. This sap, thus purified, con- 

 tains all the requisites for the formation of the wood 

 and bark of the trunk and branches, and the cuticle 

 for the formation of the leaves. 



The effect of light upon the sap is very great; 

 those plants which receive the largest amount of sun- 

 light have leaves of a deep green. Light is therefore 

 an essential element in promoting the healthy growth 



