FORM OF LEAF-TRACE 193 
or more may pass within a common sheath: a flattened vascular plate, 
or it may be a curved series of strands, is thus produced. In some leaves 
of Ferns and in some Dicotyledons the vascular tissue thus disposed in a 
curve as seen in transverse section, with its concave surface upwards, may 
show a closing in of the lateral margins as the leaf-base is approached ; it 
seems not improbable that this is connected with mechanical requirements 
consequent on the leverage of a large leaf on its base. This closing 
in may even be carried so far that the two edges may become contiguous, 
and the result will be a structure not unlike that of a cylindrical stele of the 
axis (Fig. 98). But it would be a mistake, on the mere ground of Sach 
structure and without the check of comparison, to suggest any close identity 
of character of such “pseudosteles” with the stelar condition of the axis. 
There is reason to believe that these pseudosteles of the leaf are secondary 
Fic. 97. 
Dicksonia Baromets. Portion of the vascular system of the stem, seen from within, 
and showing the departure of three leaf-traces. (After Gwynne-Vaughan.) 
in their origin, for it is the fact that they are characteristic of leaves of 
relatively large size, while smaller leaves are typically dorsiventral in their 
vascular structure. Moreover, Professor Bertrand and others have been 
able to show by exact comparative analysis that even in very aberrant 
cases of Fern-petioles the pseudostelic structure is referable still to a dorsi- 
ventral origin, and is to be explained as the result of complex foldings 
and fusions of a vand-like vascular tract. A somewhat similar explanation 
may be given of the “pseudostelic” petioles of such Dicotyledons as are 
quoted by Schoute (Zc., p 158) and of the ‘“‘polystelic” petiole of Primula 
Auricula examined by Gwynne-Vaughan ; these may be held to be secondary 
modifications of a structure originally dorsiventral, and the position may 
accordingly be summed up as follows: The construction of the axis is 
essentially cylindrical, and finds its anatomical expression in the cylindrical 
stele; the construction of the leaf is essentially dorsiventral, and it finds 
its anatomical expression in the isolated vascular strands disposed dorsi- 
ventrally. Both these are liable to modification in special cases, thus by 
N 
