DICOTYLEDONOUS STEM 



167 



The most important parts of this inner bark, so far as the plant 

 is concerned, are many tubelike structures known as sieve tubes. 



.-bark- 



.Cambium 

 layer" 



. annual 

 '"rings 



pith 

 Toys 



Explain growth in this stem. 



These are long rows of living cells, having perforated sievelike 

 ends. Through these cells food materials pass downward from 

 the upper part of the plant, where they are manufactured. 



In the wood will be noticed a number of lines called medullary 

 rays, or pith rays, radiating outward from the pith toward the 

 bark. These are thin plates of pith which separate the wood into 

 a number of wedge-shaped masses. The masses of wood contain 

 many elongated cells, which, placed end to end, form thousands of 

 little tubes connecting the leaves with the roots. In addition to 

 these are many thick-walled cells, 



which give strength to the mass of y***^ -^^^^ 



wood. The bundles of tubes with / —-S^ortex 



their surrounding hard-walled cells / ^^^W^^VJcJmbium 

 are the continuation of the bundles / 

 of tubes which are found in the I (p> pith 

 root. In sections of wood which 

 have taken several years to grow, 

 we find so-called annual rings. The 

 distance between one ring and the 

 next (see diagram) usually repre- 

 sents the amount Of growth in One Cross section of a very young dicoty- 

 r\ ±1- j. i i £ ledonous stem, showing arrangement and 



year. (jrrOWtn takes place trom a parts of the bundles and the other tissues. 



