THE STEM i6i 



tissue were between them, we should then have such 

 a stem as is shown in fig. 45, 11.^ Such in reality is 

 the structure of a very young stem of any of our forest 

 trees in its first year. The fundamental tissue in it is 

 found in the middle of the ring of vascular bundles; 

 this is the pith or medulla. We also see it in the form 

 of narrow rays diverging from the pith in between 

 the bundles ; these are the so-called ' medullary rays ' : 

 lastly, we see on the outside of this ring the so-called 

 primary cortex, generally green and sappy, and com- 

 posed of small cells. Thus every vascular bundle en- 

 closed between two medullary rays has in transverse 

 section the form of a triangle with its apex turned 

 towards the centre. 



This triangular form of the bundles is also preserved 

 in an old stem. The dark diverging rays seen in fig. 45, 

 III. represent medullary rays, and the lighter triangles 

 between them are vascular bundles. Thus the bundles 

 form the predominant part in the perennial trunk of 

 a tree ; the fundamental tissue is found in between the 

 bundles in the form of narrow medullary rays, some- 

 times almost invisible : hence it is clear also that the 

 difference between the bundles and the fundamental 

 tissue is not so well defined as in the case of mono- 

 cotyledons, and is only seen under the microscope.^ 



The principal part of the stem of our forest trees 

 consists, therefore, of vascular bundles ; this is not, 

 however, their most striking peculiarity. They differ 

 from monocotyledons, such as palms, in that they in- 

 crease in bulk during the whole of their existence — 

 which is quite impossible to the monocotyledons — 

 owing to the following anatomical arrangement. We 

 all know that the rind of our forest trees is sharply 



1 Fig. 45 I. The stem of a palm or of asparagus. II. Structure of the 

 stem of an annual dicotyledon. III. Stem of a. dicotyledonous tree. All 

 three are shown in transverse section. 



* See fig. 62, showing a smaU section of wood cut across, with a 

 medullary ray. 



L 



