John J. Skvarla

Pollen Characters in Relation to the Delimitation of Myrtales

Pollen grains representative of the Lythraceae, Punicaceae, Sonneratiaceae, Trapaceae, Oliniaceae, Combretaceae, Alzateaceae, Penaeaceae, Crypteroniaceae, Melastomataceae, Myrtaceae (including Psiioxylaceae) and Onagraceae, the twelve families constituting the order Myrtales, were examined with scanning (SEM) and transmission (TEM) electron microscopy with major emphasis on SEM. With omission of the Trapaceae, Myrtaceae, and Onagraceae, the remaining families have enough similarities to be grouped together palynologically. Heterocolpate pollen characterizes the Lythraceae, Combretaceae, Oliniaceae, Penaeaceae, Crypteroniaceae and Melastomataceae. In the latter five families pseudocolpi were noted in all taxa examined except Adelobotrys, Allomorphia, Astronia, Bredia, Oxyspora, and Tococa of the Melastomataceae and Buchenavia, Laguncularia, and Strephonema of the Combretaceae. With the exceptions of the latter four genera these taxa possess intercolpar concavities. Pseudocolpi are equal to the number of apertures except in Lythraceae where Ammannia, Nesaea and Crenea have twice the number; intercolpar concavities are also isomerous with apertures. In Oliniaceae the pseudocolpi are located in just one hemisphere and hence considered as half pseudocolpi. The Alzateaceae may have incipient pseudocolpi while the Sonneratiaceae and Punicaceae possess meridional ridges

Primitively columellaless pollen: a new concept in the evolutionary morphology of angiosperms.

Comparative study of pollen of the ranalean complex has revealed a remarkable, hitherto unrecognized characteristic of primitive angiosperm pollen, namely, its complete lack of columellae. Pollen with such exine has been desnated atectate anid taxa in the Magnoliaceae. Degenzeriaceae, Eupomatiaceae, Annonraceae, and possibly Himantandraceae and Nymphaeaceae have pollen which is considered to be primitively columellaless.

Ultrastructure of pollen exine in centrospermous families

Transmission electron microscopy was utilized to examine pollen walls of selected taxa in theAizoaceae, Amaranthaceae, Basellaceae, Cactaceae, Chenopodiaceae, Didiereaceae, Halophytaceae, Nyctaginaceae, Phytolaccaceae, Portulacaceae, andCaryophyllaceae. This conspectus is an adjunct to scanning electron microscope observations of pollen surfaces and is directed towards elucidating the basic wall structure for these families. Although differences in internal morphology were observed at the inter- and intra-familial levels, they were interpreted as reflecting variations rather than major differences. The data indicate close morphological similarities of the first ten families enumerated above, i.e., those containing betalains. TheCaryophyllaceae, an anthocyanin family, indicated a slightly greater heterogeneity of pollen ultrastructure but not to the extent of disassociating it from the betalain families. In fact, this heterogeneity was rivaled by comparable heterogeneity among and within some of the betalain families. The conclusion is that all families have close pollen morphological relationships.

THE EVOLUTION OF POLLEN TETRADS IN ONAGRACEAE

In Onagraceae, pollen is shed in mature tetrads in most Epilobieae, many species of Ludwigia (Jussiaeeae), and two closely related species of the large genus Camissonia (Onagreae). Mature tetrads of Camissonia cardiophylla and representative species of Epilobium and Ludwigia were examined with light, scanning, and transmission electron microscopes. Morphological diagnoses of monad units indicated that individual taxa could be readily distinguished. Statistical analyses of tetrads which remained after acetolysis treatment revealed significant differences in the strength of the binding mechanisms. Mechanisms of tetrad cohesion were found to consist of two principal types. Common to all taxa is cohesion of pollen wall surfaces at the aperture margins; this mechanism is well known in many angiosperm groups. With the exception of Camissonia, the remaining taxa also display binding by means of short exine fragments between adjacent pollen units. These fragments, termed bridges and reported here for the first time, are located in the area extending from the aperture margins to near the center of the proximal exine faces. Thin sections reveal that layers of the bridges are identical with those of the exine. Comparisons were made between bridges and viscin threads, both of which occur on the proximal faces of the grains. Viscin threads are present on all pollen grains in Onagraceae and exhibit distinctive morphologies, and bridges were viewed morphogenetically as related to viscin threads but including an endexine layer and occupying a position near the apertures where cohesion of wall surfaces also occurs. In an evolutionary sense, the formation of mature tetrads almost certainly occurred independently in Camissonia and may have done so in Ludwigia and the Epilobieae.

Ontogeny of pollen in Poinciana (Leguminoseae). I. Development of exine template

Abstract Development of the exine template was traced from its initiation as scattered receptors on the plasma membrane through the completed template at the end of the microspore tetrad period. Receptors for the future tectum are the first to be laid down and they occur directly on the plasma membrane before there is any indication of a primexine matrix. The primexine matrix is coincident with probacule initiation. Probacules are organized under the protectum and elongate by being assembled basally on the plasma membrane. The height of the primexine matrix increases in conjunction with growth of the probacules. Growth of the exine template terminates with the insertion of receptors for the foot layer on the plasma membrane. Poinciana gilliesii Hooker is favorable for study of exine receptors and their assembly into an exine template because of the unmistakable equivalence of the template of the tetrad and exine of the pollen grain.

Developmental origins of structural diversity in pollen walls of Compositae

Compositae exhibit some of the most complex and diverse pollen grains in flowering plants. This paper reviews the evolutionary and developmental origins of this diversity in pollen structure using recent models based on the behaviour of colloids and formation of micelles in the differentiating microspore glycocalyx and primexine. The developmental model is consistent with observations of structures recovered by pollen wall dissolution. Pollen wall diversity in Compositae is inferred to result from small changes in the glycocalyx, for example ionic concentration, which trigger the self-assembly of highly diverse structures. Whilst the fine details of exine substructure are, therefore, not under direct genetic control, it is likely that genes establish differences in the glycocalyx which define the conditions for self-assembly. Because the processes described here for Compositae can account for some of the most complex exine structures known, it is likely that they also operate in pollen walls with much simpler organisation.

Hexamethyldisilazane as a Drying Agent for Pollen Scanning Electron Microscopy

Use of hexamethyldisilazane (HMDS) as a final dehydrating solution provides robust, undistorted secondary electron images of a variety of angiosperm and gymnosperm pollen grains, including those considered to be susceptible to collapse in the scanning electron microscope. Ease of handling, low cost, lack of specialized equipment, minimal expenditure of time, and high rate of success are factors that favor HMDS over other drying agents for preparing pollen grains for scanning electron microscopy.

The Use of Osmium-Thiocarbohydrazide for Structural Stabilization and Enhancement of Secondary Electron Images in Scanning Electron Microscopy of Pollen

Abstract Pollen grains stained in a sequence of osmium (O) and thiocarbohydrazide (T) solutions (collectively known as OTOTO) appear structurally stable and undistorted in the scanning electron microscope (SEM), and usually do not require special drying. In fact, OTOTO can be regarded as another special drying method in palynology. This sequential incubation also strikingly increases the electrical conductivity of pollen grains in the SEM. Compared to standard sputter-coating or vacuum evaporative procedures, OTOTO reduces charging and yields secondary electron images with significantly higher resolution.

Electron Microscope Study of Ambrosia Pollen (Compositae: Ambrosieae)

Abstract This study presents a detailed survey and discussion of the fine structure of Ambrosia pollen. Representatives from all evolutionary groups and geographical regions are used, including 37 species and four hybrids. No significant differences are found among the species, a circumstance supportive of the congeneric nature of Ambrosia L. and Franseria Cav. hypothesized earlier from other evidence. The similarities of the pollen of Ambrosia to that of the Heliantheae and Anthemideae are discussed. Newly observed structural features of the polar and colpal regions are pointed out which support an intermediate position of the Ambrosieae between the Heliantheae and Anthemideae. Possible specializations of Ambrosia pollen within the tribe are suggested. The data also indicate that among the three closely related genera, Ambrosia, Xanthium and Hymenoclea, the relationship of Ambrosia to Hymenoclea is closest.

The elasticity of the exine.

We pressed pollen grain exines of ten genera with sizes ranging from about 20 to over 100 mm in diameter past a piston in a close ®tting cylinder. The clearance between piston and cylinder was about 20 mm. Except for exines of Betula all the other pollen types were at least twice the clearance diameter and could be expected to be greatly deformed, crushed or fractured. Cracks were evident with the light microscope in some grains and a few were clearly deformed but most appeared intact, even exines of Zea mays at a diameter of 100 ± 110 mm. With scanning electron microscopy cracks were apparent in most of the large grains (Zea, Lilium, Pinus, Crinum and Epilobium) but not in the smaller grains (Betula, Ephedra, Tulipa, Fagus and Typha). We also found many exines within exines. In some cases, e.g., Lilium, the exines entered through apertures but in other grains such as Zea and Pinus, exines came in through cracks which had opened during acetolysis or centrifugation, then closed so tightly that the cracks were dif®cult to see with light microscopy. This opening and closing of cracks in exines means that the pollen grain exine is very ̄exible and resilient and capable of withstanding shock without permanent deformation. To regain their original form the exine components that were severely cracked, ruptured or partly separated must spring back together like the partly separated halves of a tennis ball.