( Intraxylary) Phloem in the Stems of Dicotyledons. I. 581 
only ; one or two, however, have xylem completely, or in the case of others 
incompletely, surrounding the phloem. The cambium entirely surrounds 
all the bundles. The outer portion of the xylem of these bundles abuts 
very closely, only separated by one to three parenchyma elements, on 
the protoxylem of the ring-bundle. The xylem of the amphivasal bundle 
consists for the most part of short fibres with rudimentary bordered 
pits in their walls, quite similar to those of the wood of the ring-bundle. 
But the elements nearest the protoxylem of the latter are rather shorter, 
with rather thinner lignified walls, covered with very numerous simple 
pits, and with very slightly oblique end-walls. All the internal bundles 
show a tendency to doubling, their xylem consisting of two arc-shaped 
strands more or less united in the tangential plane. 
Here and there in the cylinder is 
a curious group of bundles: on one 
or both sides of one or more of its 
large bundles, a much smaller bundle 
of the cylinder occupies an oblique 
position in the angle between the large 
bundle of the cylinder and the internal 
(medullary) bundle; the latter is also 
somewhat obliquely placed. The ex¬ 
planation probably is that an attempt is 
here being made to merge the bundle 
of the cylinder and the internal (medul¬ 
lary) bundle into a more closely com¬ 
pacted and better organized whole, viz. 
into a large concentric (amphiphloic) 
bundle. 1 From the morphological point 
of view the structure indicates the onto¬ 
genetic origin of the internal-phloem 
bundles from the ring-bundles, for the obliquely situated strand represents 
one of the former which is imperfectly separated off from one of the latter. 
The arc-shape of the phloem of the ring-bundle causes the oblique position 
of the strand above mentioned; and we can find in these facts an ex¬ 
planation of the inverted orientation of the internal-phloem bundles, for if 
we imagine the oblique strand passing further towards the pith, it would, 
as we know from the analogy of similar cases, revolve further on its axis 
so as to eventually assume an inverted position. This is, indeed, exactly 
what happens when the internal-phloem strands (which represent these 
bundles in the hypocotyl) pass inwards from the phloem of the ring- 
bundle ; the phloem-strand (imperfect bundle) revolves on its axis and on 
reaching the pith assumes an inverted position. 
1 Cf. case of Acanthosicyos and the ‘ stele ’ of Primulaceae. 
Fig. 4. Citrullus ecirrhosus. Trans¬ 
verse section of central cylinder of fruiting 
peduncle, showing the independent internal- 
phloem bundles, iph , 1 internal phloem ’ ; 
sr, sclerotic zone, x n. 
