3l6 THE PRIMARY STEM 



which bend so that their shoot apices point towards 

 the open sky, and in plants growing on the edge of a 

 wood which behave in the same way. The leaves tend 

 to set themselves at right angles to the source of light 

 (diaphototropism) . As in the case of roots, the bending 

 occurs by the cells on one side of the elongating region 

 growing faster than those on the other, but the apex 

 alone perceives the light, as can be seen by covering the 

 apices of some of a crop of seedlings (e.g. of canary 

 grass) with tinfoil caps and illuminating from one side, 

 when bending no longer takes place in those which 

 are so covered. 



Etiolation. When a normally erect aerial shoot is 

 grown in the dark, it does not turn green, the plastids 

 becoming yellowish, the internodes grow enormously 

 in length and the leaves remain small and scale-like, 

 while the tissues are not properly differentiated. Such 

 a shoot is said to be etiolated. 



Structure of the Aerial Stem. The general structure 

 of the stem depends a good deal upon the fact that it 

 is essentially a leaf-bearing organ. We distinguish 

 the nodes or levels of insertion of the leaves (which may 

 arise from the stem singly or in pairs or circles, called 

 whorls), from the internodes or bare stretches of stem 

 between these levels. The stem terminates in a bud, 

 which is simply the developing apex of the shoot bearing 

 the developing leaves on its sides, and usually covered 

 by the incurving of the partly grown leaves, which arch 

 over and serve to protect the very delicate apical 

 meristem from desiccation. 



The minute structure of the outer layers of the stem 

 resembles that of the foliage leaf in more than one 

 respect. Thus it is covered by an epidermis which has 

 the same character as the leaf epidermis, and possesses 



