Structure of Acer Pscudoplatanus. 581 
tion compared with the adaxial portion of the outer ring. Their formation 
is due to further bundle displacements, and is apparently the result of the 
compression of the adaxial portion of the ring or the lateral bundles on either 
side of it (Figs. 35 A, 35 c). This compression causes an infolding or dis¬ 
placement of vascular tissue from the part affected to the inside of the ring, 
the xylem and phloem rotating during their passage inward, so that they 
come to have an inverse orientation to the system from which they are 
derived. More complex internal bundle systems of the same type often 
occur in the vascular systems of the leaves of mature plants, and earlier 
observers appear to regard the more elaborate types as an expression of 
greater size and vigour. Thus Col ( 4 ) says, ‘ Le pStiole des plus larges 
feuilles montre dans sa moelle un arc, surmonte d’un plus petit, ayant tous 
deux leur bois tourne en avant. Dans d’autres petioles, il n’y a qu’un arc 
interne. D’autres encore ont montre un tres petit nombre de fascicules 
medullaires. Ainsi, surquatre feuilles d’une pousse d’automne, toutes offrent 
au milieu du petiole, un 011 trois faisceaux liberoligneux medullaires tres 
petits.’ The size of the leaf, in the early stages at any rate, does not seem 
to us to have any direct bearing on the presence or absence of internal 
bundles, since we have noted their absence in the larger of one of the first 
pair of leaves and their occurrence in the smaller, and also their occurrence 
in seedling leaves distinctly below the average size. 
The vascular grouping prevailing at the base of the lamina is main¬ 
tained throughout the greater part of the petiole, but the adaxial portion 
often forms a continuous bar, or two contiguous bars, as a result of secon¬ 
dary thickening. The first indication of rearrangement is shown by the 
internal bundles where these are present. Although too small a number of 
petioles showing this feature were examined to enable us to make a general 
statement, it appears to be the rule that, as the base of the petiole is 
approached, the internal bundles undergo a reversal of the series of changes 
which resulted in their formation. Thus those derived from the central 
portion of the adaxial system rotate and reunite with this (Fig. 54), a gap 
being developed to admit of their insertion, whilst those derived from the 
lateral portions of the ring behave similarly. 
As the base of the petiole is approached there is a considerable increase 
in its transverse diameter, and this is accompanied by a series of bundle 
fusions which result in the passage into the stem of three bundles from each 
leaf (Fig. 40). These bundle fusions are somewhat complex, but they always 
involve two processes. These are (i) the concentration and fusion of the 
lateral bundles, and (ii) the moving outwards of the bundles constituting the 
adaxial system and their union with one or another of the abaxial series. 
The regroupings may be arranged in a series showing progressively greater 
complexity, although there is no evidence that this constitutes an evolutionary 
sequence. 
