THE VEGETATIVE STRUCTURES 77 



main zone. This small zone is called the cone dome. 

 In these genera a cone is apparently terminal on the 

 main axis, and the first cone is really terminal; but in 

 the production of a cone the embryonic tissue at the 

 apex of the plant is used up, and the growth of this axis 

 is thus brought to an end. However, a new embryonic 

 region develops at the base of the cone stalk and thus 

 establishes a new axis, which continues to bear crowns 

 of leaves until its life is brought to a close by the pro- 

 duction of a cone, and this sequence is continued as 

 long as the individual Hves. Consequently the trunk, 

 which appears to be unbranched, is really nothing but 

 a series of branches which can be seen internally, but 

 which do not appear on the surface. Each cone dome, 

 for the time being, is the apex of the plant (Fig. 24). 



The trunk of the columnar forms is easily cut with an 

 ax, but it is almost impossible to saw off a large plant. 

 It is like sawing a mass of tough cloth. An examination 

 of the microscopic structure reveals the cause of the 

 difficulty. 



A transverse section of the woody zone shows nothing 

 unusual (Fig. 25). There are the ordinary thick-walled 

 cells which one expects to find, and occasionally a few 

 thin-walled cells. The figure shows one of the growth 

 rings, the larger cells alternating with shghtly smaller 

 ones. 



Longitudinal sections, cut along a line from the pith 

 to the cortex, are more characteristic. The elongated 

 wood cells show very numerous pits of the bordered 

 type (Fig. 26), and near the pith the pits are so elongated 

 that they are long narrow shts producing a ladder-like 

 effect, so that the cells are called scalariform tracheids. 



