PLANT STRUCTURE 135 



(b) The root-cap. The root-cap is composed of cells 



beginning at what is known as the growing point, 

 and gradually increasing in size until at the very 

 tip of the root they become functionless and fall 

 off. 



(c) Cells of the vascular bundles. Beginning at the 



root-tips and passing lengthwise throughout the 

 roots and through the stem to the leaves, are the 

 vascular bundles of cells of the plant. They are 

 more or less elongated tubes. The ends of these 

 cells often become perforated, and these special 

 tubes are then spoken of as sieve tubes. 



(d) Storage cells. The cells surrounding the vascular 



bundles simply serve as storage reservoirs to hold 

 food which is not needed for the immediate use of 

 the plant. 



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



active cells which later form a part of the vascular 

 bundles. They are just beneath the bark. 



(/) Bark cells. On the outside of the stem and root 

 are cells having more or less thickened cell walls. 

 These are the bark cells. 



(g) Heart-wood cells. Heart- wood cells are cells which 

 have become practically inactive. They are 

 mostly in the interior of the plant. 



Leaf cells. Leaf cells are variously modified to suit the 

 needs of different plants. They are : 



(a) Epidermal cells. Coating the outside of leaves 

 are transparent cells, variously modified accord- 

 ing to the leaf. They are often elongated into 

 hair-like bodies. 



(6) Guard cells. Among the epidermal cells, both on 

 the upper and the lower sides of the leaves, but 



