( Para Rubber Tree): Bud- Sc ales serving as Nectaries. 223 
The view of the evolution of the shoot of Hevea that suggests itself 
to the author is as follows. Originally the base of the shoot had one, two, 
or three non-nectariferous bud-scales such as occur now ; the rest of the 
foliar organs were true foliage leaves arranged equidistantly along the axis. 
Assuming that their laminas gradually increased in size towards the middle 
of the shoots and then decreased, the lowest and highest leaves would in 
consequence be the smallest and the middle ones the largest — a condition 
often occurring in shoots. That of the Beech ( Fagus sylvatica ) is a case 
in point. Providing that the Hevea shoot had an upward tendency, as it 
has at the present day, the large median leaves would tend to overshadow 
the lower smaller ones, and thus render these latter to a great degree 
functionless as assimilating organs, and through disuse a gradual reduction 
in their laminas might follow. The nectaries on the petiolar apices still 
remaining would be the first to secrete. Viewing their service as one of 
attracting ants to keep off leaf-destroying insects, it would be an advantage 
to the plant to retain the nectaries on these retrograde leaf-structures, 
and further to increase their size and consequently their secretion, in 
order to protect the expanding foliage leaves, till their nectaries became 
functional. Thus gradually a condition which now occurs would be 
brought about. 
The Beech shoot has a few scales at its base without any lamina, 
which may be comparable, though not homologous as they are stipules, 
to the non-nectariferous scales of Hevea ; then come the foliage leaves 
increasing in size as far as the middle of the axis, and then diminishing 
towards the apex. There is a tendency in some of its shoots for the small 
lower leaves to wither and fall early. This may be partly due to their 
being shaded by the higher leaves, though this overshadowing is largely 
guarded against by the shoot as a rule having a horizontal direction, and 
as a consequence the leaf-blades are in one and the same plane. If the 
shoot, on the other hand, were inclined considerably to the vertical as in 
Hevea , then the middle leaves would shade the lower ones much more 
effectually. Such a shoot as that of the Rhododendron demonstrates this. 
It is obliquely erect and has the foliage leaves crowded together on its 
upper part, thus resembling the shoot of Hevea. The length of stem below 
the rosette of foliage leaves is not a single internode, but composed of 
several, the nodes of which in the young state were occupied by small 
leaves which have shrivelled and disappeared. These, being perhaps 
originally smaller than the middle leaves and thus subject to shade, now 
no longer persist as functional foliage leaves ; they have most likely 
decreased still further in size, and now apparently serve as protective scales 
to the bud. Consequently as a rule in horizontal shoots the lowest foliage 
leaves are the smallest, or at any rate smaller than the middle ones ; while 
in shoots inclined to the vertical the lowest leaves are the largest, because 
