Anatomy of Sarcocaulon

The anatomy of the leaf blade, petiole, stem and root of the genus Sarcocaulon (DC.) Sweet is discussed. On the basis of the leaf anatomy, the four sections recognized by Moffett (1979) can be identified: section Denticulati (dorsiventral leaves), section Multifidi (isobilateral leaves and adaxial and abaxial palisade continuous at midvein), section Crenati (isobilateral leaves, short curved trichomes and glandular hairs), section Sarcocaulon (isobilateral leaves and glandular hairs only). The anatomy of the stem is typically that of a herbaceous dicotyledon with a thick periderm. The root structure shows that the function of the root is not food storage.


INTRODUCTION
Sarcocaulon known by the common name 'Bushman's Candle' is one of the five genera belonging to the family Geraniaceae. The other four are Geranium, Pelargonium, Monsonia and Erodium. Sarcocaulon plants are fleshy short-stemmed shrublets covered with a hard waxy bark which is inflammable due to being impregnated with wax. The root system is formed by a primary root and lateral roots. Leaves are long or short petioled with the long petioles persisting as spines. The short petioled leaves occur singly or in tufts in the axils of the spines. The leaves are simple or compound with the nature of the leaf margin of taxonomic importance. Moffett (1979) divided the genus into four sections primarily on the nature of the leaf margin. The following sections and species were recognized by Moffett (1979) entirely leafless throughout most of the year. The leafless plants may produce an occasional flower (Glass & Foster, 1970), but the plants only produce leaves after sufficient rain. After such rains, the plants are capable of bursting into flower and leaf almost immediately.

M A T E R IA L AND METHODS
Material from all the species except S. inerme and S. peniculinum was investigated.
The material used was obtained from plants in their natural habitats and plants collected and planted in the Botanical Garden of the University of the Orange Free State. The material was fixed in F.A.A., dehydrated by the tertiary butyl alcohol method (Gray, 1958), embedded in Histosec wax and sectioned at 10 ^m. The sections were stained in Safranin and Fast Green and mounted in Entellan.
Leaves prepared for the scanning electron microscope were fixed in 3% gluteraldehyde in phosphate buffer (pH, 7,0), post-fixed in 1% osmium tetroxide, dehydrated in ethyl alcohol, critical point dried and sputter coated with gold.   Hm.

Lamina
Isobilateral in all the species, except S. marlothii and S. mossamedense where the leaf structure is dorsiventral. A thick cuticle is present. The adaxial and abaxial epidermal cells are of more or less the same size both containing cells of unequal size. Anomocytic stomata are found on both upper and lower surfaces of the leaf (amphistomatic leaf) and are on the same level as the other epidermal cells. The pores are irregularly orientated in surface view.
Mesophyll is differentiated into palisade paren chyma and spongy parenchyma. In the dorsiventral leaves ( Fig. 4) (S. marlothii and S. mossamedense) the palisade consists of one cell layer and is not continuous above the main vein. The palisade parenchyma is well developed and in some cases can occupy at least half the thickness of the mesophyll.
In the isobilateral leaves (Fig. 5) the adaxial and abaxial palisade consists of two cell layers with the inner palisade layer consisting of shorter cells. The adaxial palisade layer is usually better developed. In S. herrei and S. multifidum, the two examples with compound leaves, the adaxial and abaxial palisade parenchyma is continuous at the midvein (Fig. 6). In the other species with isobilateral leaves, the abaxial palisade is always interrupted at the main vein but the adaxial palisade is continuous (Fig. 7) or interrupted in the same species. In the majority of material examined the palisade parenchyma was continuous.
Vascular bundles are collateral with each vein surrounded by a parenchyma sheath. On the abaxial side of the vascular bundle the sheath cells may be sclerenchymatous. A few fibres may be present between the sheath and the phloem. The parenchy ma cells internal to the abaxial epidermis are often sclerenchymatous.
Calcium oxalate crystals in the form of druses are common in the mesophyll.
Based on the anatomy of the lamina the two sections Denticulati and Multifidi can be identified. However, in combination with the trichomes, all four sections recognized by Moffett (1979) can be identified. These are section Denticulati with dorsiventral leaves, section Multifidi with isobilate ral leaves and adaxial and abaxial palisade According to the trichomes present two of the four sections recognized by Moffett (1979) can be identified. These are section Crenati with glandular hairs and short curved trichomes and section Multifidi with glandular hairs and both types of non-glandular unicellular trichomes. In section Sarcocaulon and section Denticulati only glandular hairs are found.   continuous at midvein, section Crenati with isobila teral leaves, short curved trichomes and glandular hairs and section Sarcocaulon with isobilateral leaves and glandular hairs only.

Petiole
The anatomy of the petiole is the same in all the species. The outline is circular and a thick cuticle present. The epidermis is composed of thick-walled cells. The cortex is parenchymatous with 1 or 2 cell layers subjacent to the epidermis often chlorenchymatous. Druse crystals are common in the cortex. Three (Fig. 8) or four (Fig. 9) vascular bundles are grouped together in the centre of the petiole with the fourth bundle usually smaller. The vascular bundles are each partially subtended or completely surrounded by sclerenchyma.

Spine (thorn)
The spine is formed from the long petiole after the lamina falls off. The anatomy of the spine resembles that of the petiole except that the four vascular bundles are arranged in a circle around the pith (Fig. 10). Between each of the four vascular bundles a smaller vascular bundle is often present. The thorn forms by lignification of the cells. Lignification begins around the vascular bundles and continues from the inside to the outside. The outer cortical layers may be lignified and persistent or are cast off together with the epidermis.
The four sections recognized by Moffett (1979) can be identified from leaf anatomy. Section Denticulati with dorsiventral leaves, section Multifidi with isobilateral leaves and adaxial and abaxial  The cortex consists of parenchyma cells containing druse crystals of calcium oxalate, tannin and abundant starch grains. The parenchyma cells immediately beneath the phelloderm are rich in chloroplasts, whereas the rest are filled with starch grains. Internal to the phelloderm, a few brachysclereids are sometimes present.
With secondary growth, a fascicular and interfa scicular cambium develops (Fig. 12) The vascular cambium produces secondary xylem and phloem in radially arranged groups. Rays up to six cells wide separate the vascular bundles. Ray cells are often filled with starch grains (Fig. 13). External to the phloem, the pericycle is represented by a few fibres. In some cases, fibres are also present in the primary and secondary phloem.
palisade continuous at midvein, section Crenati with isobilateral leaves, short curved trichomes and glandular hairs and section Sarcocaulon with isobilateral leaves and glandular hairs only.
For plants growing in extremely hot dry areas, the leaves do not appear xerophytic with the stomata on the same level as the epidermal cells. Xerophytic features of the leaves are, however, the strongly developed palisade, small intercellular spaces, thick cuticle and dense covering of trichomes in some species. Although more xeromorphic features could be expected, it must be kept in mind that the leaves develop only after sufficient rain and are lost under unfavourable conditions.

Anatomy o f the stem
The anatomy of the stem is very similar in all species. The stem is typically that of a herbaceous dicotyledon with collateral vascular bundles separa ted by wide parenchymatous medullary rays. The stem is covered by a layer of cork which peels off easily. A phellogen is formed early. In seedlings the phellogen originates in the subepidermal parenchy ma while the cotyledons are still on the plant. The phellogen produces a conspicuous phellem to the outside and up to six layers of phelloderm cells toward the inside (Fig. 11) F ig . 13.-Part of a cross section through the secondary xylem of Sarcocaulon mossamedense to show the ray cells filled with starch grains. R, ray. Bar = 100 jxm.
The pith is similar in structure to the cortex.
The ability of the plants to survive the long dry seasons can probably be ascribed to the phellem, chloroplast rich parenchyma cells beneath the phelloderm, and the starch grains serving as reserve material.  and pericycle (PC). Bar = 100 pirn. REFERENCES the root consists of primary xylem in the centre surrounded by secondary xylem. The xylem appears homogeneous because of the narrow rays (1 -2 cells wide). The secondary phloem and pericycle are rich in fibres (Fig. 14) and are enclosed by the phellem.
From the root structure it is clear that the root's function is not food storage.