l6o PLANT LIFE. 



" xylein " (sap or water system), and the pipes for the 

 '' phloeml' or food material. The latter is on the 

 outside of the xylem. There is on each side of this 

 phloem a circle of living cells ; of these that on the 

 inside is "the thickening ring" or cambium^ which is used 

 to form the new xylem and phloem every year (6). 

 The other, on the outer side, is used to form new roots, 

 and sometimes to produce mechanical or strengthening 

 cells. This is the '•^ pericycle" or ^^ pericambiumr The 

 phloem is so placed, therefore, that the cambium on its 

 inside and the pericycle on its outside, will at once 

 receive all the food material brought by the phloem 

 tubes. It will also be noticed that the spokes, or 

 medullary rays, lie between the phloem patches, and will 

 thus carry the food material from the phloem inwards to 

 the pith and outwards to the cortex or rim. The water 

 or sap coming up by the xylem will also reach the 

 cambium first, and will travel, by these same medullary 

 rays, outwards, keeping the cortex and phloem fresh, 

 and inwards to the pith. 



To complete the picture, it is only necessary to put 

 in the cork-cambium, or cork-forming layer, just below 

 the cork, and often formed from the outermost cortex 

 cells. Thus there are seven concentric circles : the out- 

 side cork sheet; the cork-cambium; the cortex of which 

 the innermost layer is the " endodermis " ; the peri- 

 cycle ; the cambium ; and the boundary of the pith. 

 This, with the medullary rays crossing from the peri- 

 cycle to the pith, gives a very fair picture of the 

 average structure of a stem. 



It is necessary to have a fairly distinct general idea 

 of this structure, because the cells in a tree trunk vary 

 so greatly in appearance and in character, that it is 

 by no means easy to avoid confusion. 



