435 
Phloem in the Stems of Dicotyledons. II. 
them. The three strands, consisting of phloem only, formed, subsequently, 
connexions with the phloem of the branch-cylinder, and eventually they 
constituted the internal-phloem group of the median leaf-bundle in the 
cortex. 
The lateral leaf-bundles in the cortex possess internal-phloem strands 
which are derived from the normal phloem of the stem-cylinder. 
In C. virens , L.. medullary strands occur in some individuals, of both 
wild and cultivated specimens, in fair numbers; in other individuals they 
are quite absent. 
In C. laraxacifolia , Thuil., a cultivated specimen showed a single sub- 
amphivasal, medullary bundle at one node in the bay formed by an outgoing 
leaf-bundle; at a lower level it joined the cylinder. Two wild specimens 
showed no trace of medullary strands. 
In C. nicaeensis, Balb., and C. sibirica , L,, medullary strands are quite 
absent from the stem. 
Leaf. 
The following description relates to C. taraxacifolia : 
The vascular structure of the petiole in this species is of a rather more 
ancestral type than that of the other 
four, with which, otherwise, it generally 
conforms. That of C. biennis is shown 
in Fig. it. 
At the extreme base' is the usual 
simple arc of bundles. Higher up, 
in the typical region, a row of minute 
bundles with inverse orientation (xylem 
towards the centre) occurs, represent- 
ing the ventral or adaxial portion 
of the vascular ring. Between this 
adaxial row and the abaxial one are 
scattered minute phloem-strands, with 
here and there a bundle ; these repre- 
sent the medullary system of strands. 
There is a small central lacuna. 
All these minute bundles and 
phloem-strands, i. e. the adaxial portion of the ring and medullary system, 
arise de novo below; in other words, end blindly if traced downwards. If 
traced upwards they also end blindly at about the level where the petiole 
merges into the midrib. 
The large median bundle of the dorsal ov abaxial row is more or less 
arched and abstricts off on either side a small bundle or phloem-strand, 
with inverted orientation, on to its ventral side. 
central medullary strands ( cms ), and the 
peripheral medullary strands {pms ) ; /a, 
the lacuna, x 12. 
