STRUCTURE OF THE SHOOT 



73 



and eventually closing it up. Such a healing tissue of cork 

 is called callus. 



An interesting case occurs in leaves. Examine shoots of 

 Privet, Ash, or other common shrubs or trees in the summer, 

 and look carefuUy at the leaf -bases ; bend the leaf back- 

 wards, and note where it tends to break. Here a distinct 

 ring is clearly seen (Fig. 42, s. I), and as the leaves grow older, 

 they break off along this line very easily. 

 Observe what happens later in the season 

 and examine shoots just before the leaves 

 begin to fall. By means of a lens a 

 healing scar of cork may easily be seen 

 stretching like a plate across the leaf- 

 base. Note how easily the leaf breaks 

 off here, and also that the leaf has lost 

 its freshness, and is often torn and 

 withered, having clearly served its pur- 

 pose for the plant which bore it. In 

 compound leaves like the Common Ash 

 and Horse-Chestnut a separation-layer 

 is formed, not only across the leaf-base, 

 but across the bases of the leaflets as 

 well. 



Fig. 41 is a longitudinal section of 

 a Sycamore twig through a node, and 

 at s.l it is seen that the separation-layer 

 (or absciss layer) is already formed before 

 the leaf falls. Notice that cork has not formed across the 

 vein (v) . This is kept open to the last, for along it much 

 of the nutrient material is passed backwards into the stem 

 before the leaf is finally snapped off by the wind. 



The tissues we have considered not only form the struc- 

 ture of the shoot, but it is by their means that the work of 

 the shoot is carried on. It will be of interest to determine 

 something of this work. How and under what conditions 



Fig. 42. Part 

 of Sycamore 



Twig. /, lenticel; 

 s, leaf-scar ; s.l, 

 separation-layer. 



