100 PLANT LIFE 



may be allowed, it does not seem to matter 

 much how it is disposed. 



Every one must have noticed that the great 

 majority of our shrubs and branching trees 

 increase in girth as they get older. This in- 

 crease is produced by a specially active layer 

 of " embryonic " tissue known as cambium 

 (Fig. lie), which forms a cylindrical sheet of 

 cells situated at the outer limit of the wood, 

 which it thus completely encloses. By the 

 active division of this cambium the cylinder 

 or zone of young cell tissue is temporarily 

 rendered thicker every year, and then the 

 layers of cells which abut on the existing 

 wood are themselves differentiated into xylem, 

 to form the new annual ring of wood which 

 is added every year to the wood of the trunk. 

 A few of the outermost layers of the cylinder 

 are similarly transformed into bast or phloem, 

 and only a thin cell layer now remains as 

 a cylindrical sheet of cambium which still 

 continues to separate the wood and bast. 

 Next year this again increases in thickness, 

 and the new layers thus produced go through 

 the same changes as before. 



In this way the annual rings of wood are 

 produced which are seen when tree trunks 

 are sawn across. It is due to the still un- 

 differentiated and relatively thick sheet of 

 young cells produced every spring that the 

 " bark " is so easily separated from the 

 wood at this season. For the walls are thin 

 and the cells are rich in protoplasm and cell 



