20 THE GLORY OF THE GARDEN 



The pressure of the ascending sap is often very 

 great. When active growth begins the cells be- 

 come charged with liquid, and the pressure of one 

 turgid cell on another forces the sap onwards. The 

 best example of this force of ascending sap can be 

 seen in the case of a bleeding vine. If the pruning 

 of the vine is delayed until too late in the spring, 

 moisture will ooze out freely from the cut surface 

 of the stem, and the vine is then said to " bleed." 



Naturally, this w^eakens the future growth of the 

 vine very considerably, as, the cells having been cut 

 through, the sap escapes easily, the quantity increas- 

 ing as time goes on. 



T^he Work of the Stem. — This may be described 

 principally as " transport work." The stem con- 

 veys, by means of its vascular bundles and tissues, 

 food to the branches and leaves. And it reconveys 

 the elaborated food to those parts of the plant that 

 need to be increased in size. 



There is a continuous and definite system of flow 

 and return of food-stuffs always going on in the stem, 

 and specialised cells are concerned in that work. 



Stems vary, of course, according to the species 

 of plant under consideration; some are of annual 

 duration only, and others live for an indefinite 

 period. In the first case, the structure is of less 

 permanent or woody a nature than in that of the 

 second. 



But in all stems, whether of trees or of annual 

 plants, there is this transport work going on, a 

 definite conveyance of food-stuffs backwards and 

 forwards. 



l^he Work of the Leaves. — These may be described 

 as the workshops of the plant, wherein all the raw 



