MINUTE ANATOMY OF ROOT AND SHOOT 



213 



enemies, exposure of much green tissue to light for starch 

 making, aeration of the interior cells to allow them to breathe, 

 conduction of raw sap to the leaf and of the food substances 

 away, strength to resist winds and other strains, etc. With 

 all these needs and functions fresh in mind, the students 

 should be set to work to find out how they are arranged for 

 in the plant. 



FIG. 20. Diagram of distribution of tissues in a typical shoot, upper in longi- 

 tudinal section, lower in cross. Outer line = epidermal system ; radiating 

 lines = cortical system ; crossed lines = storage system ; spiral lines = fibro- 

 vascular system. On these systems see page 219. 



Important points to be brought out, with their reasons, are : 

 the lenticels on the stem (which are the successors, structurally 

 and physiologically, of the stomata of the younger tissues) ; 

 the greater intensity of the green on the upper, i.e. the best 

 lighted, surface of the leaf; the branching of the bundles at the 

 nodes, and the running of one branch into the leaf and of 

 another up the stem ; the fact that the bundles form a ring in 

 the stem (note the cambium, which, with the vegetative points, 



