104 



BIOLOGY 



this respect. It consists of a stem connecting two expanded 

 surfaces, the one ending in the leaves, and the other dividing 

 under the soil into fine rootlets which bear root hairs. Plants 

 obtain their food partly from the air and partly from the 

 soil, and this explains why they expand their branches into 

 leaves in the air, and their roots into root hairs in the soil. The 

 stem of the plant serves chiefly as a connection between the 

 leaf and the root and as a support for the branches and leaves. 



STRUCTURE OF THE STEM 



The structure of the stem may best be understood by begin- 



cross section of a young 

 stem, shortly after it has 



ning with the examination of 

 ep 



emerged from the seed; 

 see Fig. 44. * 



Fundamental Cells. - 

 The bulk of the stem 

 consists of a mass of ap- 

 proximately round cells, 

 which are called funda- 

 mental cells, p. These 

 cells are largest toward 

 the center of the stem 

 and grow smaller toward 

 the outer edge. The large 

 cells in the center form the pith. On the outer edge of the 

 stem is a single layer of small rounded cells forming the epi- 

 dermis (Gr. epi = upon + derma = skin), ep. Just beneath 

 the epidermis are several irregular rows of cells, larger than 

 the epidermal cells, known as the cortex (Lat. cortex = bark), 

 co. At this stage the cortex on its inner edge is not very sharply 

 marked off from the cells which fill the center of the stem, 

 and form the pith. 



Fibrovascular Bundles. A short distance within the cortex 

 will be found several groups of especially marked cells, }b, 



FIG. 44. A SECTION ACROSS THE STEM 



OF THE SEEDLING 



fb, the fibrovascular bundle; co, the cortex; 

 ep, the epidermis; p, the general fundamental 

 cells. 



