248 STRUCTURE AND VITAL PROCESSES OF PLANTS 



genous compounds are formed in the leaves. These too have to 

 be conveyed to the growing points. So the question arises which 

 way they take to descend from the leaves to their respective 

 destinations. First they travel from cell to cell to the veins of 

 leaves, and thence through the leaf-stalks to the inner bark of 

 the stem. A branch from which a ring of bark down to the 

 cambium layer is removed (fig. 228) w^ill show that the made-up 

 food does not travel in the wood in which the sap ascends. If 

 that branch is examined after one or tw^o years, it will be seen 

 that the stem above the cut has increased in size and that there 

 is a round swelling immediately above the wound. This is due 

 to an accumulation of food, the passage of which was arrested by 

 the cut. Eventually the part of the wood which is laid open 

 will probably be destroyed by parasitic organisms, so that the 

 whole of the branch above it dies. 



TJie upivard ivater-current, then, travels through the younger 

 layers of luood neighbouring the bark, and the doivnivard Jiow of 

 elaborated food travels in the inner bark adjoining the ivood. Be- 

 ticeen both of them there is the cambium layer ivliere, with the sap 

 from the ground and luith the starch from the leaves, neiv wood is 

 formed. 



4. The Covering or Protective Tissue of Stems. 



(a) The Epidermis. — As \ve have seen already on page 245, 

 young stems are covered over by a thin coat of cells, called 

 epidermis, the surface of which can be of the different descrip- 

 tions shown on page 204. The prickles of the Rose, of Rubus, 

 and Lantana are also growths of the epidermis. This protective 

 covering is sufficient for herbaceous and annual stems. When 

 it l)ecomes older, as in woody stems, the epidermis breaks up and 

 falls off in thin shreds. 



(b) The Cork. — Ihit before the epidermis is shed, a new 

 protective covering is prepared beneath the epidermis by the 

 formation of cork out of the outer cells of the rind, which obtain 

 corky walls and gradually die otf. The inner cells remain active 

 and constantly supply new cork-cells to replace those scaling oft' 

 on the outside. If the layer of cork-cells is thin, the surface of 



