Stele in Two Species of Dip ter is. 7 65 
Nothing of the kind is figured by Seward and Dale, nor has it been noticed 
in the material examined by me. 
As a result of the investigation of the different stages of the stem in 
regard to the development of the stele, we may say in general that probably 
first a protostele is formed, then the Lindsay a type occurs, and this then 
passes into a solenostele, which at first has thicker xylem on the ventral 
side, but afterwards comes to possess a xylem-ring equally developed on all 
sides. The xylem of the margin of the leaf-gaps becomes thickened, and 
higher up the stem this thickening is still more developed. There is, how¬ 
ever, a great fluctuation in development in all the tissues. They may for 
some time go back again to the condition of a less developed stage. In one 
series a certain stage may be less developed than in another, or may last 
for a shorter period. 
Roots. The roots of old plants in both species have triarch structure ; 
but the first-formed roots of both species are probably diarch. In the 
mature plants roots are given off from any part of the surface of the 
stem (Seward and Dale*) and independently of the leaf-traces. The same 
is the case in the young plants. In one case a root was even given off from 
the thinner and outwardly projecting part of the xylem-ring destined to 
form a leaf-trace. 
Petiole. The first-formed petioles have a simple structure. The xylem 
forms a flattish arch and is surrounded by phloem. In petioles which are 
a little older the xylem is horseshoe-shaped. The xylem-arch is inter¬ 
rupted by parenchyma, as seen in Fig. 2, PI. LVII, which shows the leaf- 
trace of a rather small plant while still included in the main axis. As 
already mentioned, the leaf-traces at this stage often depart with two xylem- 
strands. In petioles formed by an older plant the xylem is broken up into 
many more pieces. In the petioles which have already horseshoe-shaped 
xylem, the phloem surrounds the xylem, running in a straight line across 
the concavity from limb to limb of the horseshoe, but not lining the con¬ 
cavity. In Fig. 2 there is no phloem present at the adaxial side of the 
xylem. The part of the xylem forming the leaf-trace but still attached to 
the stele of the stem had here probably no lining of phloem on the inner 
side. In petioles formed later still we get the same characters as in the 
mature plants. The xylem is horseshoe-shaped, with the ends curved 
inwards, and the row of tracheides is interrupted by parenchyma. The 
phloem and also the endodermis follows the xylem-arch. Secretory sacs, 
which are absent in the young petioles, are present, and between them 
protoxylems are found. The investigation of the petioles of the different 
stages shows very distinctly that there is a gradual transition from the 
simple structure of the petiolar stele to the more complicated type of the 
mature plants. Thus we find some petioles which have at their base a horse- 
1 Seward and Dale, loc. cit., p. 495. 
3 E 
