152 Scott. — On some recent progress in our 
Professor Bower between the lobeliacious Rhynchopetalnm 
and the Cycads may be noticed. 
Internal phloem is characteristic of a large number of dico- 
tyledonous orders, usually, though not always, very highly 
organized ones, e. g. Myrtaceae, Onagraceae and all allied 
orders, Campanulaceae, Compositae, Cucurbitaceae, etc. Our 
knowledge of this interesting advance on normal dicotyle- 
donous structure has been much increased by the work of 
Vesque, Petersen, Weiss, Solereder, and Herail. The internal 
phloem may occur as a constituent part of the normal bicol- 
lateral bundles, as distinct phloem strands, or as a part of 
complete medullary bundles. Weiss has distinguished between 
the internal phloem of bicollateral and that of medullary 
bundles, by differences in the distribution and in the time of 
development, but in Campanula and the Melastomaceae all 
transitions occur between the three cases mentioned above. 
The internal phloem very often has cambial increase like the 
normal tissues. 
Weiss’s discovery of tertiary bundles in the xylem-paren- 
chyma of fleshy roots and Treub’s account of the adventitious 
bundles which replace the normal vascular system in Myr- 
mecodia afford striking illustrations of the plasticity of 
dicotyledonous structure. 
Closely connected with the peculiar structure just considered 
are some of the modifications of secondary thickening. This 
process in general may, I think, be regarded with advantage 
from a point of view similar to that of Alex. Braun, in his 
‘Rejuvenescence in Nature 1 .’ We are sufficiently familiar 
with the benefits afforded by secondary growth in increasing 
the area of the conducting and mechanical tissues in pro- 
portion to the increasing surface of foliage. But evidently 
this is not the whole explanation ; secondary formation of 
new elements often takes place without any immediate in- 
crease in the mere quantity of tissue. The phloem-islands 
of Strychnos go on forming new cells from their special 
cambium long after the wood has completely closed in around 
1 See p. 1 21 of the Ray Society’s translation. 
