Subapical Cells Research Articles

GROWTH AND MORPHOGENESIS IN THE CANADIAN FOREST SPECIES: III. THE TIME SCALE OF MORPHOGENESIS AT THE STEM APEX OF PINUS RESINOSA AIT.

The main shoot apex of P. resinosa is found to comprise four groups of meristematic cells constituting four generative centers by which the parts of the winter terminal bud are laid down. These are the superficial initiating cells, the group or zone of subapical mother cells, the zone of pith mother cells, and the flanking cells.The superficial primordia of the terminal winter bud pass the winter as secondary lateral budlets on the flanks of the main bud axis. Those of the leaf-bearing short shoots do not normally differentiate leaves until the following spring. Neither do those which develop into lateral long shoots. The fertile budlets, on the contrary, produce cones in the late summer and autumn. The female cone enters the winter with no tissue differentiation of the cone body. This takes place very slowly but prominently in the course of the winter. The male inflorescence is well advanced in the autumn and winter progress is relatively slight.The natural control of morphogenesis at the shoot apex including the fitful seeding habit of P. resinosa is considered in the light of the growth timetable. To explain its mechanism, a working hypothesis involving production, consumption, and concentration of auxins is advanced as a preliminary to future work.

BIOLOGY, CYTOLOGY, AND TAXONOMY OF COCHLIOBOLUS SATIVUS

Cochliobolus sativus (Ito & Kurib. ex Kurib.) Drechs. ex Dast. produced mature ascocarps on seeds of Zea mays L., previously disinfested with mercuric chloride, boiled one minute to kill the embryo and partly immersed in Sach's agar. Isolates of the conidial state, Helminthosporium sorokinianum Sacc. ex Sorokin (= H. sativum Pamm., King & Bakke), and monoascosporic isolates were hermaphroditic and heterothallic. Uninucleate, spherical spermatia 1.7 μ wide, the sole male agents in plasmogamy, were produced in phialids in slimefilled spermogonia measuring 2.5 × 2.5 mm. Hyphal fusions and conidiation did not initiate the heterokaryon in the ascogonium. Homokaryotic isolates and incompatible crosses produced partially mature ascocarps by differentiation of maternal stroma. Pseudoparaphyses extended downwards from subapical stromatic cells and produced locules. Subsequently in compatible crosses the heterokaryotic ascogenous hyphae grew from the sub-basal pseudoparenchyma and produced a fascicle of asci in the locule. Eight scolecospores, uninucleate at inception, formed after meiosis. The asci had two walls (bitunicate) but the inner wall was not evident in asci filled with spores nor functional in spore discharge. Whole asci extruded from the ascocarp in slime and dehisced in water by a terminal circumscissile rupture. Cochliobolus Drechs. was placed in the Pseudosphaeriaceae Höhn. sensu Luttrell in the Pseudosphaeriales Thiessen & Sydow.

TWO NEW HOMOPTERA FROM AFRICA, AND SYNONYMICAL NOTES

Scaphoideus Anneœ, sp. nov.—Different from all the other species of Scaphoideus known to me, by the presence of only one median subapical cell in the tegmina, instead of two: the subcostal (marginal) cell widens apically, the outer branch of the raidal vein being continuous up to the apex of the tegmen, not ending at the base of teh subapical cells as in the other species. The interolateral margins of the eyes are straight, diverging very slightly towards the dorsal apex, and the posterior margin of the pronotum is a little more emarginate.