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Endospermogenesis in hydrophylleae hydrophyllaceae in relation to taxonomy

Endospermogenesis in hydrophylleae hydrophyllaceae in relation to taxonomy

Kurtziana 19: 13-34

This study deals with the early development of the endosperm in species of the five genera included in the tribe Hydrophylleae of the Hydrophyllaceae. The main conclusions are: 1. The endosperm formation is ab initio cellular and belongs to the isopolar type and transversal subtype (1212) of the EODP system; in other words, the first and the second divisions are horizontal, producing a four celled row (cfr. DI FULVIO and COCUCCI, 1986). 2. After the second mitotic cycle, the development of the endosperm follows two divergent lines: a) In Eucrypta, each of the four cells divides vertically, resulting in four tiers of two cells each. The two cells of the micropylar tier do not divide again, but extend themselves around the zigote forming a long, bicellular haustorium; these cells never produce a lateral branch and their nuclei are small. The other three tiers of cells, give rise to the storage endosperm, but some derivatives of the upper one, turn also haustorial. The chalazal haustorium is absent; b) In Ellisia, Nemophila, Hydrophyllum and Pholistoma, cell divisions are confined instead, to the two middle cells of the four-celled row, which are responsible for the origin of the storage endosperm. The terminal cells remain undivided and function directly as two unicellular and well developed haustoria. The chalazal haustorium gives out one prominent lateral branch (or two in Hydrophyllum) which grows towards the funiculus and sometimes penetrates the placenta. This usually grows in the opposite direction in Hydrophyllum, except when two branches are formed; in this case, one of these grows towards the funiculus. The micropylar haustorium forms a lateral branch only in species of Hydrophyllum and Pholistoma, growing always towards the placenta. In both haustoria the nucleus is hypertrophied; in case the haustorium is branched the nucleus stays close to the the end of the branch, excepting the chalazal haustorium of Hydrophyllum where the nucleus does not move to any branch. 3. The movement of the nucleus inside the haustoria, is associated with strands of fibrous material, probably microtubules, arranged between the nucleus and the end of each branch. From the taxonomic point of view, the peculiarities of the endosperm development leads to the following conclusions: 4. Eucrypta should be segregated from Hydrophylleae; its endosperm development is similar to the one present in Phacelieac, the tribe where this genus should be transferred. The unique feature of bearing ovules on both axial and abaxial faces of broad parietal placentae, keeps also Eucrypta somewhat apart from the other genera of Phacelieace, but the alternative solution of setting up a separate group, must await further studies of Phacelia and allied genera. 5. The tribe Hydrophylleae should be organized with Hydrophyllum, Pholistoma, Nemophila and Ellisia. 6. On the basis of cytological and palynological similarities, coupled with the ones of endosperm development, the evolutionary relationships of Hydrophyllum seem to be much closer to Pholistoma than they are to the genus Nemophila, with which Ellisia is more related. 7. The results of this study on Hydrophylleae indicate that the endosperm development, like pollen morphology and chromosome number, bears considerable taxonomic and perhaps even phylogenetic importance within the family Hydrophyllaceae, and might afford additional evidence for determining its affinity with other families. The peculiarities of the early endosperm development, support the inclusion of the Hydrophyllaceae in Solanales sensu Cronquist (1981) and in the Dilleniid-Leafed Asterids proposed by Hickey and Wolfe (1975). The possibility also exists of a relationship between Hydrophyllaceae and families of the Ericales and Dipsacales in the sense of DAHLGREN et al. (1981), as it was illustrated by Yamazaki (1974) in his system of Gamopetalae based on the embryogeny.

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Accession: 005359826

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