138 THE PRINCIPLES OF FLORICULTURE 



wise through the root to the stem and to the 

 leaves. These are vascular bundles. They 

 serve as direct passages by which crude food 

 substances pass from the root-hairs and the epi- 

 dermal cells of the root to the cells of the leaf. 

 Here, in the presence of light and air, the foods 

 are changed into such forms as the protoplasm 

 of the plant can use. The vascular bundles 

 then conduct the prepared food back from the 

 leaf to the active, growing cells of the root-cap 

 and to other vital parts. The cells which con- 

 duct the crude sap to the leaves are near the 

 center of the stem, while those which return the 

 prepared foods are near the bark. 



(d) Storage cells. If more food is prepared than the 



growing cells demand, the surplus food passes 

 into cells in the immediate vicinity of growing 

 parts and is there stored until it is needed. 

 These storage cells are located near the cambium 

 cells of the root and trunk. 



(e) Cambium-layer cells. Cambium-layer cells are 



located just beneath the bark. Here active 

 growth in thickness of the root and stem takes 

 place. The cambium layer runs through the 

 center of the vascular bundles, and is, therefore, 

 near the food supply. The cells increase rapidly 

 in size, and with it, rapid cell division takes place, 

 and growth results. Growth in thickness of the 

 stem, therefore, only occurs directly beneath 

 the bark. Any injury to the bark of a plant 

 interferes with healthful growth. 



(/) Bark cells. Bark cells are on the outside of the 

 growing tissue of the vascular bundles. They 



