Monographs Details: Orectanthe
Authority: Carlquist, Sherwin. 1960. Anatomy of Guayana Xyridaceae: Abutboda, Orectanthe and Achtyplola. Mem. New York Bot. Gard. 10: 63-117.
Family:Xyridaceae
Scientific Name:Orectanthe Maguire
Discussion:

The single section shown (fig. 103) will serve to indicate the nature of the seed coat in this genus. The number of layers outside of the inner two may be greater or fewer than that shown, but is never fewer than three cell layers and usually more, depending on ring formation. The outermost layer of cells consists of thin-walled sclereids. A number of thin-walled but persistent cells are present between the epidermis and the inner pair of cell layers. The persistent tissue is distinctive only in the presence of intercellular deposits, which occur in irregular patches and are of the same material Avhich fills lumina of the innermost pair of cell layers of the seed coat. The inner two cell layers are distinctive, as in Abolboda, in their occlusion with the resin-like or tanninlike contents. In origin and nature, these cell layers are precisely homologous with those two layers in Abolboda. The outer of these layers does not exhibit the remarkable alteration into ridges which occurs in Abolboda, however.

Comparison of Seed Coats in Xyridaceae.

Unfortunately, no seeds of Achlyphila distich a were available. There are descriptions of seeds of Xyris in the literature, although only for one species, X. indica (Weinzieher 1913; Netolitzky 1926). These descriptions show a layer of cells with dark contents, formed into longitudinal ridges on the seeds, which has sculpturing on the outer wall surface and appears to correspond with the similar layers with contents in Abolboda and Orectanthe. Weinzieher, however, suggests that the origin of this layer is from the inner layer of the outer integument, rather than the outer layer of the inner integument, as is the case in Abolboda. Careful comparative studies of ovules in Abolboda and Orectanthe showed that the outer thin-walled layers, without contents, are derived from the outer integument, whereas the inner pair of cells with occluded lumina are derived from the inner integument. From very casual observations on seeds of Xyris, the writer is of the opinion that the respective layers in Xyris actually have a similar origin, and that the interpretation of Weinzieher is open to question.

The differences in seed structure between Abolboda and Orectanthe appear to be very distinctive expressions of the same basic pattern, and may be used both to separate the genera and to emphasize an underlying similarity. The writer believes that seeds of Xyris, upon further study, will probably also reveal similar conspicuous variations on a pattern common to the whole family. Study on seeds of Achlyphila is needed for the same reasons.

Endosperm.

Weinzieher (1913) and Netolitzky (1926) indicate rounded compound starch grains and protein spheroids in endosperm of Xyris. Presumably both of these accounts are based on Weinzieher's study of X. indica. All the species for which material was available of Abolboda (fig. 101) and Orcctanthe showed these features. Immature stages in the endosperm of .1. sprueei showed the multinucleate condition of endosperm cells very clearly.

Embryo.

Weinzieher (1913) has illustrated stages in the development of the embryo of Xyris indica. The embryo consists of relatively few cells. A broad, flat cotyledon is evident, but Weinzieher figures no differentiation of a suspensor or any meristematic region. Features like these occur in Abolboda, as illustrated for A. americana in figure 101. A basal and a hypoba^al cell are evident, however. The cotyledon consists of large cells, with dark-staining contents that suggest the haustorial function of this structure which is closely appressed to the endosperm. At the edges of the cotyledon, flap-like extensions are evident. The embryo of Abolboda americana shows no differentiation of an apical area, and is larger and composed of more numerous cells than that of X. indica. Significantly, the embryo of Orectanthe seeptrum (fig. 102) shows precisely the same features as the embryo of Abolboda americana, but is larger, corresponding to the larger size of all org-ans in this species. A basal and a hypobasal cell appear to be present. If a meristematic area is present, it is not appreciably differentiated from other areas of the embryo. Unfortunately, no embryos of Achlyphila were available. Likewise, no stages in gametogenesis or early embryo development were visible on account of the nature of preservation.

POLLEN

Methods

Pollen grains of Xyridaceae are subject to decomposition by various reagents. As Erdtman (1952) notes, his acetolysis method degrades the exine. Sodium hydroxide, a reagent commonly used in pollen-preparation techniques, partly or wholly dissolves the exine. Except for two specimens (figs. 112, 120) in which NaOH treatment was very gentle and merely revealed some textural differentiation in the exine—probably because of preferential solubility of certain parts—this substance proved useless. Even the pollen of fixed material which formed the main basis for this study was, in some cases, altered. Because of this extreme sensitivity to a wide variety of reagents, the simplest methods for preparation of pollen grains for observation appeared to be (1) sectioning and staining of flowers (with their included pollen) according to the techniques described at the beginning of this paper; and (2) transferring fixed anthers through an alcohol series to absolute ethyl alcohol, staining in safranin dissolved in absolute ethyl alcohol, and following- this by transferring anthers to xylene and making wThole mounts of grains in Canada balsam. Undoubtedly both of these techniques resulted in dehydration and perhaps other changes which could alter size and possibly induce artifacts. However, a number of grains in each of the collections studied appeared to be reasonably unaffected, and were considered suitable for study. Note should be made of the fact that the spines (or other excrescences of the exine) that stain bright red are seemingly unchanged by any of the reagents mentioned, and data concerning these are without doubt quite reliable. The exine wall, which stains green with a safranin-fast green combination, is the structure that is sensitive to reagents.

Because the grains are large, and few in number per anther, because good material (grains from anthers just before or at anthesis) were necessarily limited, and because many grains were collapsed, or degenerate, the number of grains that could provide reliable measurements was extremely small. The data on pollen grain diameter below represent, therefore, averages of measurements of only those grains that the writer judged to be turgid and otherwise unaltered in form. On account of the exceptional susceptibility of pollen grains in this family to various changes, truly exact data can probably be obtained only from fresh material. Such material is virtually impossible to view because of the remote locations in which these species grow. Terminology for pollen morphology used below follows the usage of Erdtman (1952).

Pollen Grain Size.

Pollen grains of Abolboda, Orectanthe, and Achlyphila are sphaeroidal, a fact which may be related to their nonaperturate condition. Because of this shape, only one dimension is necessary. Dimensions do not include the spines or other excrescences.

Within the genus Abolboda, some taxa seem obviously to have larger pollen grains than others; species with exceptionally large grains include A. linearifolia, A. macrostachya (at least in part), A. grandis, and A. sprucei. The pollen grains of Orectanthe are markedly larger, in general than those of Abolboda. The markedly smaller diameter of Achlyphila pollen grains is interesting; this size is comparable to the sizes reported by Erdtman (1952) for species of Xyris. Pollen grains of Xyris, as described below, do differ in morphology from those of Achlyphila.

Previous reports on pollen-grain diameter include those of Malme (1933) on ''Abolboda" (now Orectanthe) sceptrum ("about 105 µ") and Erdtman (1952) for Abolboda poarchon ("105 µ"), A. pulchella ("105 µ"), and A. vaginata ("at least 75 µ"). Maguire, Wurdack et al. (1958) give the following measurements: Abolboda bella, 120-140 µ; A. paniculata, 140-150 µ; and Orectanthe (as a genus), 160-250 µ.

Pollen Grain Wall ("sporoderm").

Sectioned material showed two distinct parts of the pollen grain wall in all taxa: the inner portion, which stains brightly with fast green, and the spines, or other ornamentation (including the minute pila on the surface of the interspinal areas), which stain bright red with safranin. These two portions appear to correspond with Erdtman's definitions of nexine and sexine respectively, except that a thin layer, which stained pale green, on the inner surface of the pollen grain wall in Abolboda could be interpreted as the intine. This interpretation was suggested by observation of germinating pollen grains on a style of A. grandis. In these, this layer covered the emerging pollen tube. Such a layer could not be distinguished in pollen grains of Orectanthe, possibly because of difficulties in preservation, but a comparable layer did appear present in grains of Achlyphila (fig. 128).

The nexine in sectioned material and in whole mounts appears to stain homogenously in most preparations. In two preparations in which sodium hydroxide had been employed (fig. 112, 120), radial fibrillae or striae appeared to be present in the outer two-thirds of the nexine. This appearance may have been caused by differential solution of materials in the nexine, or by expansion of the wall by means of the treatment. Among the various taxa, the nexine thickness appears to be a distinctive character. In most of the Abolboda species studied, it appears to range between 5 and 7 p. It is much thicker than that in A. grand/is var. guayanensis, A. macrostachya var. macrostachya, A. niacrot achy a var. robustior, and A. sprucei. Certainly the great thickness of the nexine of A. grandis var. guayanensis and A. sprucei represents a distinctive characteristic. In Orectanthe, the nexine does not appear to exceed 5 fi in thickness. In Achlyphila, the width of this layer is about 3 µ.

The outermost layer of the pollen grain wall, or sexine, is basically a thin membranous layer which may be partly detached in damaged grains. Minute pila (seen in surface view, figs. 113, 129) are embedded in this membrane. A feature that seems quite important to the writer does not seem to have been mentioned in the literature on Xyridaccae pollen grains, namely, that these minute pila are, in fact, homologous with the large spines or knob-like ornaments in Abolboda and Orectanthe. These larger excrescences are deposited on the membranous layer just as the pila are, and appear to be composed of the same material, despite the vast difference in size between the two types of ornamentation on the same grain. Rarely, as in Orectanthe sceptruin (fig. 125), knobs transitional in size between these two categories may be observed. The pila in Orectanthe are noticeably larger than those in Abolboda. Pollen grains of Achlyphila (figs. 128, 129) show the largest pila of any of the grains studied here. In Achlyphila, the pila tend to be aggregated in flake-like patches. In places between these patches, the pilate layer is absent. In Abolboda, a thin hyaline layer was observed immediately beneath the pilate layer. Such a layer may also occur in pollen grains of Orectanthe and Achlyphila.

The spines on pollen grains of Abolboda are so characteristic of that genus, and are so subject to distinctive variations in the species, that they have been figured for all the species studied (figs. 106-120). Basically, as in A. acaulis or A. linearifolia, they have an inverted funnelform shape. Sodium hydroxide treatment (figs. 112, 120) reveals that lacunae, not visible in other preparations,, are probably present in the spine base. Some species show markedly narrow spines: A. bella (fig. 108), A. acicularis (fig. 107), and A. ciliata (fig. 109). Very short, wide spines occur in A. ebracteata (fig. 110) and A. macrostachya var. robustior (fig. 120). The three varieties of A. macrostachya appear different on the basis of spine size and shape. Within a single flower of A. sprucei, various alterations in spine shape were noted. The basic condition seems to be that shown in figure 114, but multiple spines on a single base (fig. 115), very much reduced lobes (fig. 116), and merely the vestige of the spine-base (fig. 117) were also observed.

Erdtman (1952) has illustrated the spines of pollen grains of three species and given dimensions: A. poarchon (spines 8.3 µ long; basal diameter 5.5 µ), A. pulchella (spines 9.3 µ, long; basal diameter 8.3 µ), and A. vagineita (spines 8.3 µ long; basal diameter 8 µ). Erdtman figures spines in the latter species which appear about like the spines in A. macrostachya var. angustior.

The comparable ornamentation on pollen grains of Orectanthe is generically different from that of Abolboda. In Orectanthe, the emergences take the form of a large knob, or other shape much wider and more blunt than the spines of Abolboda, with the possible exception of the much smaller structures in A. sprucei. In O. ptaritepuiana (figs. 121-124) a variety of shapes was noted, varying with the collection from which the grain was taken: short, with a rounded apex (fig. 121), elongate (fig. 122), with a curved apex (fig. 123), or with folds or bulges on the sides (fig. 124). Evidently considerable variation is present in these characters. In preparation of pollen grains in this species, the knobs appeared to be sunken in slight depressions; this may well be an artifact of fixation or preparation. The knobs of Orectanthe seeptrum pollen grains are different from those of 0. ptaritepuiana in that the apical portion is widened, so that a capstan-like shape is achieved. The only variation in structure of these knobs is found in the formation of additional small knobs on the base (fig. 127) as compared to the normal condition (fig. 125), or the formation of a roughened base (fig. 126).

Discussion.

In summary, the pollen grains of the three genera show excellent generic characters. The presence of large excrescences in Orectanthe relates this genus closely to Abolboda, but the differing shape is a good generic character. Likewise, the larger pila in the surface between emergences differentiates Orectanthe from Abolboda. The absence of such large emergences in Achlyphila, and the grouping of pila into flake-like aggregations on the exine surface, marks this genus off from other genera of Xyridaceae. Within Abolboda, differing exine widths and distinctive spine shapes and sizes serve to differentiate the species, so that of the species studied, most could be identified by means of the pollen grains alone. Similar considerations apply to distinctions between the two species of Orectanthe.

Erdtman (1952) has emphasized the great distinction between Abolboda and Xyris in pollen-grain structure. Much of this distinction may be attributed to the large spines on the surface of Abolboda pollen grains. The lack of such larger ornaments on grains of Achlyphila, which are (like those of Abolboda, but unlike those of Xyris) nonaperturate, may vitiate this contrast, and provide an intermediate form. The pilate layer in pollen grains of Abolboda, Orectanthe, and Achlyphila probably has a counterpart in Xyris. The presence of the pilate layer, although different in each genus, tends to provide a common characteristic, as does the rather thick nexine, lacking in lipophilic compounds. The writer suspects from Erdtman's reports of both "OL" and "LO" sexine patterns that a layer of united pila may compose the sexine in at least some Xyris pollen grains. The exine of Achlyphila seems midway between that of Abolboda and Orectanthe on one hand, and Xyris on the other. The relatively thick, baculate sexine and the thin nexine of Xyris pollen grains do not seem at odds with the condition in Achlyphila. Probably the three major patterns represent striking variations on the same basic pattern. Careful work is needed both to provide additional data on Xyris and to demonstrate whether the affinity Erdtman (1952) suggests between the grains of the Abolboda-type and those of Liliaceae and Zingiberaceae is close or not.

SUMMARY

The writer has found that an attempt to summarize anatomical characteristics of genera of Xyridaceae in chart form is virtually impossible because (1) there are exceptional species within a genus; (2) reference to particular conditions in lengthy anatomical terms, or with the aid of illustrations is necessary; and (3) some relatively subtle characteristics cannot be summarized in this manner. The most important anatomical lines of evidence are cited below, and the reader is referred to the foregoing descriptions, particularly to the "discussion" section terminating each of the major portions above.

Abolboda. Characteristics of leaf anatomy and stem anatomy suggested recognition of two species-groups, termed here "smaller-stemmed" abolbodas and "larger-stemmed" abolbodas. ^Recognition of these as subgenera would probably be premature. Within each of these groupings, leaf and stem anatomy suggest specific characteristics also. Characteristics other than these do not emphasize the two species-groups as much as they suggest characteristics of individual species. The large stele of roots defines A. macrostachya, but genuinely remarkable specific characteristics are offered by the peculiar patterns of endodermis-cell thickening. In the inflorescence axis, thickness of selerenchyma ring, presence of fibrous sheaths on pith bundles, and selerification of epidermal cells offer a number of characteristics. Distinctive patterns in number of bundles, size, selerenchyma and chlorench.yma presence and distribution offer further characteristics in sepals, and, to a lesser extent, in bracts. Perhaps the best specific characteristics are evident in pollen in such respects as shape and size of spines, nexine thickness, and diameter.