The trichomes consist of basal-, stalk-, and apical cell. The cuticle
proper is lamellate, but extremely thin on apical cells. Exposed walls
of stalk cells and the sunken basal cells develop thick cuticular layers.
A lamellate suberin wall is deposited all over the inner surface of
the mature stalk cells, cutting off plasmodesmata. Then the apical cells
are shed. This happens when repeated hygroscopic spiralizing and despiralizing
cause brittleness in the wall between the apical cell and the stalk
cell. The trichomes develop before differentiation of stomata begins.
The primary function of the indumentum is to enhance the transpiration
stream, thereby securing sufficient mineral supply for rapid leaf growth.
Secondary, the indumentum may protect against high insolation during
leaf growth. Although the hairs are dead on mature leaves, they are
of no value in reducing transpiration due to a wide spacing. Therefore
the apical cells can be shed early. Trichomes with tannin-storing apical
cells were observed mainly in the axils of young leaves. Possible ultrastructural
differences between lamellate cutin and suberin walls are discussed.