Anatomy of Ter ato logical Seedlings. IV. 481 
whole forming a mesarch xylem completely surrounded by phloem (Fig. 95). 
Meanwhile the phloem of the double bundle has bent inward and the two 
halves have united adaxially (Fig. 95). The two strands thus formed 
traverse the greater part of the hypocotyl unchanged, but near the base the 
phloems open out on the inner side and unite laterally, the mesarch bundle 
at the same time becoming exarch and causing a break in the phloem at 
this point also (Fig. 96). Two lateral roots are developed and a new proto- 
xylem originates in the plane at right angles to these (Fig. 97), so that the 
tap-root is triarch. The interpretation of the singular vascular behaviour 
of this seedling is distinctly difficult, though it shows points of considerable 
resemblance, especially in the concentration towards the middle line of the 
lateral strands, to a similarly odd seedling previously described by one of us 
( 18 ). The division of the midrib as the petiole is approached suggests that 
not only have the two phyletically distinct midribs merged their identity, but 
that one of the lateral strands has also become involved in the intimate union. 
In the absence of further supporting evidence, however, such an explanation 
must be regarded as purely tentative, and the chief interest of the seedling 
is that it provides a further instance of the plastic nature of juvenile vascular 
structures. 
Discussion. 
The investigation of the structure of seedlings, such as those described 
in the present paper in which a pseudo-monocotyledonous condition is 
a relatively common teratological feature, provides a tempting starting-point 
for a consideration of the possible origins of the Monocotyledons proper. 
Theoretically the seedling possessing a single leaf is derivable from the 
dicotyledonous type in a variety of ways. These may be briefly tabulated 
as follows : 
1. By syncotyly involving both cotyledonary margins (amphisyn- 
cotyly) so that a cotyledonary tube, often more or less solid secondarily, 
results. This condition is occasionally realized in Impatiens Roylei. 
2. By ^ syncotyly involving one margin only of the originally 
separate cotyledons. This is the condition most frequently found in 
the syncotyls of I. Roylei. 
3. By a specialization in function of the cotyledons, one of which 
becomes purely suctorial, the other undergoing retardation in develop- 
ment and subsequently appearing as the first leaf. 
4. By a total suppression of one cotyledon. 
5. By the suppression during embryogeny of the meristematic 
activity of the cotyledonary apex, the other cotyledon, with the basal 
sheath common to both, continuing to develop and producing the 
typical sheathing structure which characterizes the majority of Mono- 
cotyledons. 
