1921] JEFFREY & TORREY—HERBACEOUS ANGIOSPERMS 13 
outer region of the pith corresponding to the medulla of the 
secondary axis are several amphivasal concentric fibrovascular 
bundles, precisely similar to those so commonly found in the stem 
of monocotyledons. The amphivasal strands are the result of the 
fusion of the confronting bundles of main axis and lateral branch 
by their phloems. This condition is not an uncommon one for 
dicotyledonous stems of marked herbaceous nature, particularly 
when they bear large leaves with many foliar traces entering the 
axis at the nodes. Other examples of this feature will be supplied 
later. 
Fig. 24 shows a section of the stem of a buttercup at some dis- 
tance below the node. A rather old stem has purposely been 
chosen, so that the identity of the foliar traces in the stem may 
readily be distinguished. In such stems, in the later summer, 
gummosis invades the cavities of the vessels of the foliar traces, a 
feature which makes the leaf traces stand out even in low magnifica- 
tions. To the extreme right is seen the median trace of the leaf, 
and above and below on either hand are the lateral traces, two on 
either side. The foliar traces are emphasized by their blackness, 
resulting from the mucilaginous contents of their vessels. Fig. 25 
is a portion of a section similar to that illustrated in fig. 24, some- 
what more highly magnified. It is now quite easy to discern that 
the two outside bundles have vessels plugged with darkened con- 
tents, while the vascular structures in the central bundles appear 
quite clear and devoid of gummosis. Fig. 26 furnishes a further 
example of the phenomenon of gummosis in the external bundles, 
which here as in the other instances are foliar traces. The leaf 
trace to the right is much smaller than its counterpart on the left, 
and represents in fact an extreme lateral trace of the leaf. 
In fig. 27 appears a part of a stem preserved in the early summer, 
in which as a consequence gummosis is not present in the vessels 
of the foliar traces. The magnification is greater than in the two 
preceding figures, and. more details of the structure of the fibro- 
vascular bundles are evident. Obviously the two larger bundles 
at the ends present a different structure from those toward the 
center. This expresses itself in a more parenchymatous organiza- 
tion of the xylem, in which comparatively few vessels are distributed 
