170 STEMS 



forms bark only. The cork 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 some- 

 times 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 like the layers of wood. It is stripped 

 off in sheets, which are sold in the market as cork. Cork 

 is very impermeable to water. It is this quality, along with 

 its pliability and lightness, which gives it commercial value. 

 Its cell walls have become thickened withsuberin (su'ber-in), 

 a substance somewhat similar to the cutin of which you 

 learned in connection 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 cylindrical arrangement in annual stems differs 

 from the cylindrical 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 little 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 



