THE PRODUCTION AND DIFFERENTIATION OF SECONDARY XYLEM IN XANTHIUM PENSYLVANICUM

The removal of leaves and buds from the shoot of Xanthium seedlings caused the cessation of xylem fiber differentiation in all internodes while allowing the production of cambial derivatives to continue toward both the xylem and the phloem. The potential xylem fibers developed into parenchymatous cells with thin cell walls. The vessels developed normally except for their small size. Cambial derivatives and vessels were produced linearly with time in intact plants (6.1 cells per file/day and 9.7 vessels per vascular bundle/day) and in decapitated plants (2.2 cells per file/day and 5.5 vessels per vascular bundle/day). Fiber production was linear with time in intact plants (163 fibers per vascular bundle/day) and did not occur in decapitated plants. When a single leaf was allowed to develop from a lateral bud of a decapitated plant, xylem fiber differentiation was restored for a period of time corresponding to the period of rapid expansion of the leaf blade. When the leaf passed the phase of rapid expansion, it no longer had an inductive effect on xylem fiber differentiation.     

IS POLYPLOIDY NECESSARY FOR TISSUE DIFFERENTIATION IN HIGHER PLANTS?

Measurements of relative DNA per nucleus of cells from various tissues show that cell differentiation can occur in the absence of polyploidy in higher plants. In Pisum polyploidy was present in roots, sepals, pods, pistils, and stamens but not in petals or leaves. In Triticum cells of leaves exhibited some polyploidy, but no polyploid cells were present in mature roots. No polyploid cells were found in any tissue of Helianthus examined (roots, cotyledons, stems, leaves, sepals, petals, pistils, and stamens). Therefore, as a general rule, polyploidy should not be considered essential in tissue or organ differentiation of higher plants. In Helianthus polyploidy is unnecessary for the completion of the life cycle.     

A因子在原核生物分化和次级代谢中的分子调控

发育分化是现代生物学一个重要的前沿研究领域。近二十年来,原核生物细胞的发育与分化取得了令人瞩目的成就,如芽孢杆菌的内源性孢子形成,放线菌的分生孢子分化,粘细菌的多细胞形态发生等。形态分化与次生代谢之间的关系是目前的一个研究热点,本文介绍近年来Ａ因子及其类似物在原核生物分化和次级代谢调控中的研究进展。１　Ａ因子及其类似物１９８２年,Ｈａｒａ等人分离到一株丧失了生物合成链霉素和孢子形成能力的灰色链霉菌（Ｓｔｒｅｐｔｏｍｙｃｅｓｇｒｉｓｅｕｓ）突变株,发现当提供一种由野生型菌株产生的可扩散因子时,突变…     

HYBRIDIZATION,POLYPLOIDY AND ADVENTITIOUS GROWTH IN THE GENUS ASPLENIUM

Gametophytes of Asplenium platyneuron, A. rhizophyllum and A. monlanum were grown from spores under controlled conditions. Factors inhibiting and promoting germination and growth were determined. Leaves cut from sexually produced sporophytes of A. platyneuron, A. rhizophyllum and A. montanum were cultured under controlled conditions on agar slants. The leaves produced neoplastic growths of two kinds: (1) two-dimensional growths, (2) three-dimensional growths. The former developed into normal-appearing, rhizoid- and gametangia-producing, heart-shaped gametophytes having a diploid chromosome complement. The formation of two-dimensional growths was favored by conditions drastically reducing the energy supply of the initiating sporophytic cells and by destroying the correlation of the leaf. Three-dimensional growths most often developed into sporophytes unless conditions were limiting; in the latter case they transformed into two-dimensional growths. Both two- and three-dimensional growths developed from fragmented leaves excised from buds growing near the tips of acuminate fronds of A. ebenoides. The high frequency of two-dimensional adventitious growth leads to the conclusion that apospory may provide a mechanism for the production of fertile allotetraploids, by the fusion of diploid gametes of adventitiously produced diploid gametophytes, in the genus Asplenium and perhaps other fern genera, in contrast to that which has been previously suggested in the literature.     

AUTORADIOGRAPHIC STUDY OF RNA SYNTHESIS DURING SECONDARY RHIZOID CELL DIFFERENTIATION IN PTERIS VITTATA

The pre- and postmitotic stages of rhizoid cell differentiation in Pteris vittata L. were pulsed in radioactive uridine for 30 min and the data analyzed quantitatively by autoradiography. The total grain count rises from 594 in the nondifferentiating cell to 1,369 in the corona stage; increases to 2,745 in the internally segmented rhizoid cell (IRC) and to 2,779 in the wheel; and then drops to 1,306 in the protrusion and to 1,261 in the early rhizoid. The significant increase in nucleolar labeling in the corona reflects an increase in ribosomal RNA synthesis for mitosis-associated syntheses. The two-fold increase in grains over the cellular compartments of the IRC and wheel reflects an increase in RNA synthesis in these postmitotic stages. The similarity of labeling profiles between these stages suggests that they are not far enough separated to show a difference in their roles. The decline in total grain counts in the protrusion and early rhizoid stages to half the maximum levels suggests that an overall decrease in RNA synthesis is attended by a shift from a differentiating phase to a steady-state growth system.