3.3 Hydathode anatomy and distribution
The anatomy of hydathodes in the Crassula species examined was
rather conserved (Fig. 6 ). They all displayed the anatomical
arrangement typical of epithemal hydathodes, with three main components:
water pore(s), epithem and tracheids. The water pores consisted of more
or less sunken pairs of guard cells, which were partially concealed by
subsidiary cells. The epithem appeared as a mass of small
achlorophyllous cells (Fig. 4 ) underneath the water pore(s),
with cytoplasmic contents such as nuclei noticeably stained
(Fig. 6 ). A loosely arranged array of xylem tracheids irrigated
the epithem and connected it to the leaf vasculature. Cells containing
tannins were observed in association with the hydathodes (Figs.
4, 6 ), forming a more or less continuous sheath in some species (e.g.C. ovata ). Anthocyanin contents in hydathode sheath cells were
observed in C. perforata and C. ausensis (Fig.
4 ).
Two types of hydathodes (sensu Martin and von Willert 2000) were
observed (Table 2 ). Type I hydathodes, characterized by being
larger and having numerous water pores within a more or less circular
epidermal area, were observed in C. multicava and C. ovata(Figs. 5, 6 ). These hydathodes were also associated with a
mineral crust, observed with both light and electron microscopes
(Figs. 3–5 ). The remaining species exhibited type II
hydathodes, which tend to be smaller and with a single large water pore,
often with bulging subsidiary cells (Figs. 4–6 ). The water
pores of these hydathodes were located, if present, within the
aforementioned papillary/trichomic/idioblastic clusters and/or surface
tubercles. The foliar distribution of hydathodes varied among species
(Table 2 ), yet they always occurred more abundantly, or even
exclusively, on the surfaces most exposed to the atmosphere.