The genus Acacia Miller in South Africa . I . Stipules and spines

A large num ber of seedlings an d young twigs o f Sou th African Acacia species was sectioned a n d the vascularization o f the nodes and in ternodes studied. T he nodes o f all the species exam ined are trilacunate and the vascular tissue o f the stipules or iginates from the lateral leaf traces. T he G um m ife rae species all have spinescent stipules, while stipules o f the Vulgares species a re m em branous. Prickles conta in ing no vascular tissue are found on the nodes and in som e species also on the in te rnodes o f the Vulgares species. These prickles always occur on the ridges form ed on the stem by leaf traces. IN T R O D U C T IO N The genus Acacia in South Africa is represented b y only two of the six subgenera or series recognized by Bentham (1875), viz. the Vulgares and the G um m i­ ferae. The Gummiferae species all have spinescent stipules and no other spines or spinescent outgrowths occur on the stem, while the Vulgares species have membranous stipules together with prickles at the nodes and/or on the internodes. Vassal (1969) describes the external morphology of the stipules and spines in seedlings of African Acacia species. Nothing, however, is mentioned about the nodal anatomy or the vascularization of the seedlings. Vassal also bases the phyllotaxy of these seedlings on their external morphology alone. A study of the stipules and spines of the South African Acacias cannot be regarded as complete without a good knowledge of the vascularization of the nodes and leaf bases. It was therefore necessary to study the amtomy of the stem, in particular the anatomy of the nodes and the vascularization of the whole seedling axis. M A TE R IA L S A N D M E T H O D S Seeds of all the South African Acacia species (see lera p. 476-*79 for list of all specimens used in this and following studies, series I-V) were germinated and seedlings were fixed in F.A.A. after they had reached the fourto eight leaf stages. Serial sections were made of the root, hypocotyl, cotyledonary node and nodes of the first two vegetative leaves. For the study of the nodal anatomy of young twigs on mature trees, material of A. caffra (Robbertse 851, PRU) was used. R E S U L T S A N D D IS C U S S IO N Sketches of the serial sections of the seedling axis of A. kraussiana from the root to the second vegetative leaf are shown in Fig. 1. Apart from differences in length of the internodes and other small differences, the basic anatomy of seedlings of the different species is the same. In Fig. la, the first vegetative leaf is sectioned through the petiole, while the second leaf is sectioned through the pulvinus. In the pulvinar region, the vascular tissue of the leaf forms a closed cylinder. Proximally, the cylinder breaks up to form the three leaf traces, Fig. Ic-e. From the lateral leaf traces vascular tissue branches off to the stipules (Fig. If and g). Fig. I .— Line d iagrams o f a scries o f cross sections o f the seedling axis o f Acacia kraussiana, taken from a ttachm ent of the first two vegetative leaves, dow n to the root. hi. 1, first vegetative leaf; hi. 2, second vegetative leaf; st, s tem; st. bl., stipules; w, x, y, z, vascular bundles o f the stem. ann fibres

The genus Acacia in South Africa is represented by only two of the six subgenera or series recognized by Bentham (1875), viz. the Vulgares and the Gummi ferae.The Gummiferae species all have spinescent stipules and no other spines or spinescent outgrowths occur on the stem, while the Vulgares species have membranous stipules together with prickles at the nodes and/or on the internodes. Vassal (1969) describes the external morphology of the stipules and spines in seedlings of African Acacia species.Nothing, however, is mentioned about the nodal anatomy or the vascularization of the seedlings.Vassal also bases the phyllotaxy of these seedlings on their external morphology alone.
A study of the stipules and spines of the South African Acacias cannot be regarded as complete without a good knowledge of the vascularization of the nodes and leaf bases.It was therefore necessary to study the amtomy of the stem, in particular the anatomy of the nodes and the vascularization of the whole seedling axis.

MATERIALS AN D M ETH O D S
Seeds of all the South African Acacia species (see lera p. 476-*79 for list of all specimens used in this and following studies, series I-V) were germinated and seedlings were fixed in F.A.A. after they had reached the four-to eight leaf stages.Serial sections were made of the root, hypocotyl, cotyledonary node and nodes of the first two vegetative leaves.For the study of the nodal anatomy of young twigs on mature trees, material of A. caffra (Robbertse 851, PRU) was used.

RESULTS A N D DISCUSSION
Sketches of the serial sections of the seedling axis of A. kraussiana from the root to the second vegetative leaf are shown in Fig. 1.Apart from differences in length of the internodes and other small differences, the basic anatomy of seedlings of the different species is the same.
In Fig. la, the first vegetative leaf is sectioned through the petiole, while the second leaf is sectioned through the pulvinus.In the pulvinar region, the vascular tissue of the leaf forms a closed cylinder.Proximally, the cylinder breaks up to form the three leaf traces,   In all the Vulgares species, the ground tissue of the stipules, as seen in the serial cross sections, fuses with that of the stem before it fuses with the ground tissue of the leaf base.In all the Gummiferae species the ground tissue of the stipules fuses with the leaf base before the latter joins the stem.
The nodes of all the leaves are trilacunar and the "stele" of the stem consists of four collateral vascular In the lower part of the hypocotyl the vascular cylinder breaks up into four vascular bundles.The primary xylem of each bundle again divides into two groups.Each of these primary xylem groups turns through 180° in the direction of the nearest medullary ray where it pairs with a second group of primary xylem originating from an adjacent vascular bundle.Thus a tetrarch stele is formed (Fig. 1 r-u).bundles.The central leaf tra:e of leaf No. 2 enters the "stele" of the stem between vascular bundles y and z (Fig. lg), while the two lateral leaf traces enter through the leaf gaps between vascular bundles w and y, and x and z respectively.The central leaf trace of leaf No. 1 enters the "stele" of the stem between vascular bundles xw and xz, while the lateral traces enter through the leaf gaps between bundles y z and xz, and xw and y respectively (Fig. 2).
It is clear from this investigation that the first and second leaves are alternate, even though, organographically, they may appear to be opposite or sub opposite.
Below the nodes of insertion of the cotyledons, the vascular tissue of the axis divides into two sections, forming two lacunae through which the vascular tissue of the cotyledons enters the stele.In Fig. 1 m-q the cotyledons have been removed, but the two leaf traces from each cotyledon can be seen entering the lacunae.This region is, in fact, a compound node consisting of two monolacunar nodes, each with two leaf traces.Vassal (1969) has dealt with the ramifications of the vascular tissue in the cotyledons and this will therefore not be repeated in this paper.
The same type of vascularization as in the nodes of the first and second vegetative leaves is found in the nodes of the third, fourth and succeeding leaves of the seedling and in nodes of young leaves on mature trees.The resulting phyllotaxy is 2/5 (Fig. 2).In this type of vascularization there are five prominent vascular bundles in the internodes and, apart from the leaf traces, four prominent bundles in the nodes.The five prominent bundles in the internodes form five ridges on the surface of young stems.Three of these ridges below each node lead to three leaf traces (Figs. 2 and 3).In the Vulgares species where prickles are found, they occur only on the ridges.In Acacia schweinfurthii, A. kraussiana and A. brevispica the prickles occur in five longitudinal rows along the five ridges on the internodes.The same type of distribution of prickles is also found on seedlings of A. hereroensis, young coppice shoots of A. here roensis and A. caffra. In A. caffra and A. hereroensis, however, prickles are also found on the nodes.
In most other Southern African Vulgares species two prickles are found on the nodes (Fig. 3), while in the case of A. Senegal, three prickles are found at each node.In species where two prickles are found, they originate on the ridges leading to the two lateral leaf traces (Figs. 3 and 4).In A. senega!with three prickles at each node, they originate on the ridges leading to the three leaf traces.In no South African Acacia species have prickles ever been found between ridges as they are always situated on the ridges.Prickles on the nodes do not always appear at the same level, but one may be placed lower down on the internode and in some cases one or both may be lacking.Prickles are often found on the leaf rhachis and rhachillae of certain species, but even in these cases they occur opposite the extension of the central leaf trace into the rhachis or ramifications of the central leaf trace.
No vascular tissue is found in the prickles (Fig. 4).They consist only of ground tissue covered by an epidermis.
In the Gummiferae species and A. atbida the spines are modified stipules.As in the stipules of the Vulgares species, stipules of Gummiferae species are supplied by vascular tissue branching from the lateral leaf traces (Fig. 3f and 4).The stipules of the Vulgares species are caducous, but those of the Gummiferae species develop into long, straight or curved persistent spines.
Spines of A. erioloba (-A.giraffae sensu auct.mult, fide Ross, 1975) are inflated at their bases and A. luederitzii var.retinens also develops occasional inflated spines.Monod & Schmidt (1968) have commented on the pseudo-galls found in African Acacias, but in this study, no further attention has been given to this aspect.
The vascularization of the leaf in South African Acacia species will be dealt with in a subsequent paper.
Fig. Ic-e.From the lateral leaf traces vascular tissue branches off to the stipules (Fig. If and g).

Fig
Fig. I .-Linediagrams of a scries of cross sections of the seedling axis of Acacia kraussiana, taken from attachment of the first two vegetative leaves, down to the root.hi. 1, first vegetative leaf; hi. 2, second vegetative leaf; st, stem; st.bl., stipules; w, x, y, z, vascular bundles of the stem.

•
Department of General Botany, University ofPretoria.

II 1 Fig 2 .-
Fig 2.-The phyllotaxy and vascularization of a yong stem of Acacia caffra I, schematic diagram of the primary vascular bundles; II, diagram of the phyllotaxy.1-6, trilacunar nodes.

Fig. 3 .
Fig. 3.-Line diagram of a series of cross sections through the petiole and node of Acacia caffra.C, section of proximal part of petiole; D. E, section of pulvinus; F, G, H, section of node, fl, phloem; gb, glandular body; Ivb, lateral vascular bundle; oks, vascular tissue of axillary bud; svb, central vascular bundle; v, fibre cap; vts, vascular tissue of stipule; x, xylem.

Fig. 4 .
Fig. 4.-Schematic reconstruc tion of the course of the primary vascular tissue in the node of the stem of Acacia caffra.Small letters, a h, refer to Fig. 1 and capital letters, A H , to Fig.
ACKNOW LEDGEM ENTS I am indebted to the Council for Scientific and Industrial Research and the Research Committee of the University of Pretoria for financial assistance.Afrikaanse Acacia-spesies is gemaak en die verloop van die vaatweefsel in die knope en litte is bestudeer.Die knope van al die ondersoekte soorte is trilakuner en die vaatweefsel van die steunblare ontspring uit die laterale blaarspore.AI die Gummiferaesoorte se steunblare is dorings terwyl die steunblare van die Vulgares-soorte membraanagtig is.Haakdorings wat geen vaatweefsel bevat nie kom op die knope en by sekere soorte ook op die litte van die Vulgaressoorte voor.Laasgenoemde dorings word steeds op riwwe op die stam, wat deur die blaar spore veroorsacik word, aangetref