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THE ESSEX NATURALIST.
plants growing on land at the pond's edge, are deeply cut into
fine segments while later floating leaves are circular and broadly
lobed. Another instance of heterophylly occurs in our Water-
lilies, where the broad submerged leaves are flaccid and thin in
texture, and the floating leaves circular or oval and of much
firmer consistency. It has been found that a well-developed
floating leaf of the Yellow Water-lily contains about eleven
times the dry weight of a submerged leaf of the same area.1 An
even more striking example is seen in the Arrow-head (Sagittaria
sagittifolia) of our slow streams and rivers, where the first formed
submerged leaves are long, flaccid and ribbon-like, while the air
leaves are firm, erect, and either oval or arrow-shaped.
Dr. Arber has pointed out in her book Water Plants (p. 156)
that the submerged type of leaf appears to be in reality the
juvenile form, but it can be produced later in the life-history in
consequence of poor conditions of nutrition; the air leaf, on the
other hand, is the product of the plant in full vigour and
maturity. We may note that the aquatic and air leaves are
distinguishable from one another in the submerged bud.
Among woody plants a contrast between the juvenile and
mature form of leaf is characteristic of many species of Conifer.
For example, the Japanese Cedar, Cryptomeria japonica, has
soft spreading foliage on the shrub-like young plant, so very
different from the stiff erect foliage of the mature tree that the
juvenile form was long regarded as a distinct species, and called
Cryptomeria elegans; the stiff type of foliage is well suited to
withstand exposure to sun and wind, while the juvenile foliage
of the sheltered and often shaded bush is fitted to take advantage
of all available light. Among Cypresses and some Junipers a
similar contrast between the juvenile and mature types of
leaf has led to the plants with juvenile foliage being regarded
not only as distinct species but as belonging to a distinct genus,
Retinospora, and the distinction is still adhered to by most
nurserymen. It is interesting that cuttings taken from the
Retinospora form always retain the juvenile form of foliage.
Passing from Conifers to woody Phanerogams, we are familiar
with the seedling plants of Gorse and Broom, with their trefoil-
leaves, characteristic of members of the great family of Legum-
inosae to which they belong, while the later leaves, formed on
1 Arber, A. Water Plants, p. 259.