in the Root and Stem of Dicotyledons ; 287 
capable of producing phloem only without wood. All such 
arbitrary limitations of the possibilities of dicotyledonous 
structure are rash. Until Sanio’s discovery in Tecoma , no 
one would have suspected the existence of medullary cambium 
at all. Now we know that it is of common occurrence, and 
that whether it be connected with bicollateral or with inde- 
pendent medullary bundles, it is alike able to form xylem or 
phloem or both, according to the special requirements of the 
plant. The only safe generalization as to cambium is that it 
may arise in any living tissue, to whatever 'system’ this may 
belong, and that it is capable of producing any form of tissue 
for which at the moment the plant has an increased demand. 
4. Gentiana acaulis , L. Our observations on this plant, to 
which reference has already been made, may be described 
very shortly, for they are in close agreement with those of 
Meyer and Jost, above cited, on other species of the genus. 
In the stem of this species the bulk of the phloem is internal. 
Its arrangement is unusually complex. In the older inter- 
nodes we find two concentric rings of internal phloem-groups, 
in addition to which there is often a third set lying quite in 
the middle of the pith. At first sight one is disposed to 
regard these strands as forming an independent medullary 
system, but investigation of their longitudinal course shows 
that this is not so, but that all the internal strands are 
branches from the inner phloem of the bicollateral bundles, so 
even in this extreme case we see no sufficient ground for 
departing from De Bary’s terminology. 
The main bundles of the leaf are bicollateral. The leaves 
themselves are opposite, decussate, and sheathing. Three 
bundles enter the stem from each leaf, but fuse into one broad 
bundle before turning down into the internode. The fused 
bundle runs down through two internodes, its elements spread- 
ing themselves out tangentially 1 , and ultimately joins the 
outgoing bundles of the leaf vertically beneath, i. e. at the 
1 As Meyer says of G. lutea , the elements separate so widely that one can no 
longer speak of the course of the bundles, but only of the connection of the 
elements, loc. cit., p. 501. 
