PROBLEM 2. 



The Part Stems ajtd Roots Play in Making Food 



153 



x'Vlong the outer portion of the vas- 

 cular cyhnder lie tissues known as phloevi 

 (flow'-em). In the phloem are several 

 kinds of cells; those of special interest 

 are the sieve tubes. A sieve tube is a row 

 of long narrow cells which remain alive. 

 In this respect sieve tubes are different 

 from the xylem tubes. Holes appear in 

 the walls at the top and bottom of each 

 cell so that these walls look like sieves. 

 Strands of cytoplasm pass through the 

 sieve from cell to cell. 



Between the phloem and xylem lies 

 a very narrow ring of thin-walled cells, 

 the cambiimi cells, which also remain 

 alive. They, unlike the phloem or the 

 xylem, have the ability to divide. The 

 cambium ring is so narrow that it can- 

 not be seen without the aid of a micro- 

 scope. 



What is the structure of stems? All 

 roots are very much alike in their gen- 

 eral structure but there are two distinctly 

 different kinds of stems. The monocot 

 stems are quite different from the dicot 

 stems. First let us study the stem of a 

 dicot shrub or tree. How does this stem 

 compare with the root? It has the same 

 cylinders as the root with which it con- 

 nects: the epidermis, the cortex, and the 

 vascular cylinder. 



The cortex of most stems, being above 

 ground where some light reaches it, has 

 cells that are green. As the stem grows 

 older, just as in roots, cork tissue is 

 formed from cortex cells just inside the 

 epidermis. This cork tissue cuts off the 

 water supply from the epidermis and 

 kills it, leaving cork on the outside. The 

 stomata which are present in the epi- 

 dermis become lost and new openings 

 through the cork, called lenticels, form. 



Fig. 177 Note 

 the boles in 

 the sieve 

 plate ivhich 

 lies between 

 the tipper 

 and lower 

 cell. Sieve 

 tubes carry 

 manufac- 

 tured food 

 through 

 plants. See 

 pages /57- 

 ijS. 



Cork is not formed in the stems of some 

 herbs. In most herbs it is only a thin 

 layer, but in many woody stems the 

 cork layer becomes quite thick. The 

 cork oak that is native to Spain forms a 

 cork layer that may become several 

 inches in thickness. It is this tissue that 

 is cut into cork stoppers. In some trees 

 new layers of cork form, first from cor- 

 tex cells nearer the phloem and then from 

 the phloem cells themselves. All the orig- 

 inal cortex cells and then the older 

 phloem cells thus are cut off from a 

 supply of water and die. The bark of 

 such trees is rougrh and furrowed. The 

 cortex of birch and certain other trees 

 lasts for a long time. Their bark is smooth 

 except in old trees. In beech the cortex 



