I70 STEMS 



appears in the outer bark, and which forms bark cells 

 only. This cambium has no connection with the inner 

 cambium we have just been describing. It produces cork 

 cells and produces them only on its outer side. It is the 

 outermost layer of the cortex, or sometimes a deeper one, 

 which is transformed into this cork cambium. The cork 

 from which corks for bottles are made comes from the 

 cork-oak, a tree which is abundant in Spain. The outer 

 bark of the cork-oak is very thick and is formed in layers 

 hke the layers of wood. It is stripped off in sheets, which 

 are sold in the market as cork. Cork is very impermeable 

 to water. Its cell walls have become thickened with 

 suberin (sil'ber-in), a substance somewhat similar to the 

 cutin of which you learned in c6nnection with your study 

 of roots. 



In view of the continued growth of the wood, it is not 

 difficult to understand the cracks and furrows which appear 

 in the outer bark. It is constantly being ruptured by 

 pressure resulting from the growth of the wood within. 



The cyhndrical arrangement in annual stems differs 

 from the cyHndrical arrangement in perennial stems 

 chiefly in the fact that in the former the vascular bundles 

 are more widely separated than in the latter. Sometimes 

 they are not even connected by extensions of the cambium. 

 Sometimes, in stems of this kind, the cambium does not 

 appear at all. Since such stems die at the end of their 

 first growing season, evidently they have Httle need for 

 cambium and the secondary growth which cambium 

 causes. For their purposes primary growth is often suffi- 

 cient. Stems of this kind are usually either pithy or 

 hollow. The stem of the sunflower is a good example of a 

 stem of this type which is pithy ; asters and golden rod and 



