ANATOMY OF TISSUES. 115 
succeeding years’ growths no ducts are formed, their place being 
taken by tracheids. 
As might be inferred from the character of the open bundle, 
which is normal only to the dicotyledonous type, these plants 
are designed to prolong the growth of their main axis in all 
three directions, while plants whose stems have closed bundles 
increase in diameter only until the bundle system has’reached 
its full development. After this time the stem increases in 
diameter only in a few exceptional forms. This continued in- 
crease in the diameter of dicotyledons and gymnosperms is of 
such a character that it can best be treated in a separate chapter. 
The process by which it is accomplished is generally termed 
secondary growth, and this takes place in root as well as stem. 
It may therefore be omitted until the description of the general 
structure of the root has been given. 
The normal method of branching in all stems above the 
vascular cryptogams may be described as axillary ; that is, the 
branch originates in the axil of the leaf. Those branches occur- 
ring elsewhere on the stem are termed adventitious, there being 
no law governing their place of appearance. It is not possible 
here to enter into a description of the various methods of growth 
of these secondary organs, but it may be said in general of those 
that are normal that their bundle systems originate in the 
periblem of the stem tip, and like the bundles of the leaves 
usually from the outer layers of the meristem. For this reason 
both sets of organs are called exogenous in referring to their 
place of origin. The mode of attachment of their bundles to 
those of leaf and stem varies greatly, but it takes place in such 
a manner that not only is there a direct continuity of the main- 
stem and side-branch bundles, but in the greater number of 
instances the pith of the main stem immediately joins that of 
the side branch. For a more complete discussion of this subject 
the reader is referred to De Bary’s Comparative Anatomy and 
also many more recent monographs. 
