COURSE OF TRACHEAL TUBES THROUGH STEM 103 



lary rays, they are needed above by unfolding buds and devel- 

 oping seeds and fruits. 



The spiral and annular thickenings are such a nice adjust- 

 ment to specific conditions that their significance, already men- 

 tioned in Chapter II, will admit of further discussion. Tubes 

 of these kinds are the first permanent tissues to become differen- 

 tiated from the procambium, and they will be found in stem cross 

 sections not far from the growing apex and on the side of the 

 procambium bordering the pith (Fig. 14). Already before other 

 tissues of the vascular bundle make their appearance in the 

 procambium strand these tubes have characteristically thick- 

 ened and lignified their walls.' In longitudinal sections they 

 stand out sharply as channels for conduction of water into the 

 meristematic region close to the growing apex. Where these 

 tubes appear a good deal of stem elongation has yet to take 

 place, and it will be seen at once that wall thickenings in the 

 form of rings or spiral bands will allow the tubes to stretch 

 without interposing much resistance to stem elongation. 

 The pitted kinds of tubes which later appear are formed 

 farther back from the apex where growth in length has 

 ceased, and these are stronger and better adapted for long 

 service than are the earlier sorts which finally become ruptured 

 and ineffective. 



Course of Tracheal Tubes Through the Stem. -The 

 course of the tracheal tubes through the stem is best followed 

 by tracing the course of the vascular bundles of which they 

 form a part. Nearly all vascular bundles end in the leaves, 

 and only rarely do they end in the stem without entering a leaf. 

 Therefore there is no better way to trace the bundles than to 

 begin in the leaves and follow their course downward. Pro- 

 ceeding in this manner we find that the bundles or bundle, as 

 the case may be, that descend into the stem from each leaf con- 

 tinue their downward course through one or more internodes, 

 and then as a rule branch and fuse with bundles that have entered 

 the stem from ot^ier and lower leaves (see Figs. 49 and 50). In this 

 way the whole system of bundles in stems is made continuous, a 



