66 



DOMESTIC BOTANY. 



ing the leaves of any flowering plant to which of the two 

 divisions it belongs. But in all systems of plant classi- 

 fication there are many exceptions to the general rule ; 

 in the present case the Yam and Sarsaparilla families, 

 hold an intermediate position between exogens and 

 endogens, being bramble-like, leafy shrubs, with reti- 

 culated veins, but connected with endogens by having 

 only one cotyledon. 



Sap and Secretions of Plants. — All parts of a plant 

 contain a fluid called sap, which, like the circulation of 

 blood in animal life, maintains the vital action of vege- 

 table life ; its colour and nature diff'er considerably in 

 various plants. In the birch, maple, and vine it is clear 

 and limpid ; in poppy, euphorbia. India-rubber, and cow 

 trees it is white, like milk ; in celandine and gamboge 

 tree, yellow ; in the blood-tree of Norfolk Island, red ; in 

 aloes and terebinths, green, and becoming black or purple 

 when exposed to the air. Its constituents furnish pro- 

 ducts of great importance, as sugar, opium, India-rubber, 

 gutta-percha, turpentine, gums, &c., which are obtained 

 either by natural exudation, pressure, distillation, or by 

 incisions. In the latter case the sap runs out, and may 

 be considered analogous to bleeding, and although this 

 process is periodically repeated in many trees, yet they 

 do not appear to suffer from the loss, being again re- 

 plenished. 



Much has been written on the flow of the sap, and 

 curious phenomena have been observed ; but here it must 

 suffice to explain only its presumed mode of action. 

 Take for example all trees that shed their leaves in 

 autumn, as the plane, elm, lime, &c. ; in winter they may 

 be compared to dormant animals, which on the increasing- 

 warmth of spring start into active life. The sun, by 



