Many of the embryological characteristics that may be useful, when employed judiciously and in conjunction with other characters, in arriving at taxonomic conclusions are listed Several characteristics that show a non-random distribution between families of the Magnoliatae and Aose of the Ljhatae are discussed Features which are more predominant in the monocotyledons than dicotyledons are: monocotyledonous type of development of the anther wall;^oebo?d tapetum; successive cytokinesis of the microspore mother cells to form isobilate al teTraoV helobial endosperm development; and a single cotyledon in the mature embryo. In contrast' those characteristics that are more prevalent among dicot families than monocot families include basic and dicotyledonous patterns of anther wall formation; simultaneous cytokinesis of ^0! spore mother cells with the formation of tetrahedral tetrads; hemitropous, ampin ropous or circmotropous ovules; ovules with a single integument; an endotheliuni; OeJhera PeLa Peperorma Plumbago or Plumbagella types of megagametophyte development; cellulaf eX S^tW °°*l S m ^ matUre embrya In Spite ° f these differenced those characteristics that are most common-occurring in at least 70%, and usually more, of all angiosperms -are evenly distributed between the two classes and afford a strong embryological unity foX angiosperms. These widespread characters include: four microsporia per lither^feLti! ated endothelium; two-celled pollen grains; bitegmic anatropous ovules" Polygonum t£>e of megagametophyte development; and nuclear endosperm. Within the two classes distributon of £ exlZV r 1Sn °l ^^ ?, r0 P° rtional am ^S tibe several subclasses and superb for example a unitegmic, tenumucellate ovule with an endothelium, cellular endosperm with haustona and Solanad embryogeny predominates in the Ericanae and Asteridae; or helob al J± S™ ^ a d t St ir tlVe ^^ °f *? A^dae. On the whole, embryological characteristics are remarkably constant at the family level. In those families where variation does occur, genera are usually constant, although a few notable examples of mtragenerS and even T3&XT? d ° 6XiSt ' " 1°J eXam ^V n PattemS ° f -gagametophyte dLTopment cateeorie^ofSrl ^T™? usef ^ess of the grosser aspects of embryology-the major de afk wtthl I ^ or f devel °P m e nt -^me evidence is presented suggesting that variation in nSZ^T^ g cate f or y> such as size > shape, and cellular characteristics of the developing SnerT ^ megagamet0phyte ' may be helpfuI fa det emiining relationships within famUies, gcucid, or species.

No abstract

Development of the introrse, tetrasporangiate, and normally dorsifixed and poricidal stamens has been studied at the gross morphological and cellular level in ten species of Ericaceae. Microsporogenesis, followed in four species, is normal, with cytokinesis simultaneous, forming tetrahedral tetrads. The tricolp(or)ate pollen is shed as permanent tetrads with each segment two-celled except in Enkianthus in which pollen grains are three-celled monads. Anther-wall development is similar in all four species initially, but no regular pattern of wall development could be recognized thereafter. The tapetum, of parietal origin, is binucleate, glandular, and mainly uniseriate. Viscin threads occur with the tetrads in the three rhododendroid species. A well-developed endothecium appears only in Enkianthus.Soon after stamen initiation, anthers of nine species invert at the eventual filament-anther junction to become introrse; in Enkianthus inversion occurs close to anthesis. Microsporogenesis starts during early inversion; greater cell elongation on the abaxial side of the young anther completes inversion by the late sporogenous-tissue stage. In Erica and, to a lesser extent Calluna, inversion results from greater abaxial than adaxial increase in cell number and length just above the filament-anther junction. The single vascular strand reflects the degree of inversion. Stamens of six species are appendaged; three have only awns, two only spurs, while one has both. Appendages arise from residual meristems after inversion is completed (or almost so) in all except Enkianthus. Awns develop at what will be the apex at maturity of each anther half. Their length and orientation vary among species. Only in Vaccinium do the awns become hollow (tubules). Spurs, varying in length, shape, and size, arise on the abaxial side from the filament, connective, or thecae.

The floral morphology and anatomy of one representative of the Parnassioideae and two of the Brexioideae are described, and some of the recent literature dealing with the Saxifragaceae sensu lato is reviewed. Comparison of the floral structure in Parnassia to that typical of the Saxifragoideae, the subfamily constituting the Saxifragaceae sensu stricto and which, therefore, may be considered to show the basic saxifragaceous characteristics, reveals little similarity. Parnassia differs in pattern of both sepal and androecial vascularization, vascularization and degree of connation of the carpels, height in the gynoecium to which ventral bundles remain compound, possession of nectariferous staminodia, and the absence of epidermal appendages. Brexia and Ixerba (both of the Brexioideae) are strikingly dissimilar in floral structure and probably should be dissociated. While the position of Ixerba is problematical, it shares more floral characters with the Escallonioideae than with either Brexia or the Saxifragoideae and is better associated with that taxon. In both Parnassia and Brexia the vascular pattern suggests derivation of the androecium from a fascicled condition: the vascular supply of each filament consists of a cylinder of closely associated collateral bundles, and each staminodial set receives a single vascular complex which subsequently divides into as many vascular strands as there are staminodia in the set.