CHAPTER III 



THE STEM 



31. The Function of the Stem\ — The stem has for its chief 

 function the production and display of the leaves and roots and 

 the conduction of the materials which these organs art- especially 

 concerned in handling. It serves as a connection between them, 

 carrying up the material absorbed by the roots and distributing 

 the various substances received from the leaf. Like the traffic 

 of a city this material is received at many st.it i< »ns and trans- 

 ported along various channels to many points. At one place 

 some of it is used in the manufacture of food, at another point 

 material is required for the nourishment and construction <>t' the 

 cells. Here a portion is stored, or again, there is the useless 

 or waste material to be carried away. These movements are 

 nol to be considered as currents of water flowing now in this, 

 n<>w in that direction. Jusl as rool hairs take up fluids from the 

 soil and force them out into the adjacent cells, SO each cell in the 

 path of transport is constantl) receiving material and forcing it 

 out again into the surrounding tissue. 



32. Structure of the Stem. In order to comprehend the nature 

 of the work performed by the stem it will be necessary to ex- 

 amine its structure and note the character of the apparatus uti- 

 lized. If thin sections are made across the stems of seedlings, 

 as of the castor bean, or of shoots just emerging from the buds, 

 the tissues will resemble the arrangement shown in Fig. 39. 

 All ver\ young stem- of dicotyledons, i. «\. plants whose seed 

 lings have two leaves, as the squash, the bean, etc., show more 

 or less clearly three regions : an epidermis, a cortex, and a central 

 region surrounded 1>> the cortex, an arrangement alread) noticed 

 in the roots. The epidermis does not materially differ in struc- 

 ture, modification, or function from that already noted in the 

 leaf, see page 8. The cortex is largel) compos,-, 1 of paten 



