Roots growing under low water potential commonly exhibit a marked decrease in growth rate and in diameter. Using median longitudinal sections of fixed maize (Zea mays L. cv WF9 x Mo 17) seedling roots, we investigated the cellular basis for these effects. Cortical cells in the shortened elongation zone of water stressed roots were longer than cortical cells in the comparable location of well-watered roots. Nearly twofold differences in cell length were seen in the region 2 to 4 millimeters behind the root apex. The shortened growth zone, however, leads to a final mean cortical cell length approximately 30% shorter in the stressed roots. These differences were present regardless of the age of the control roots. These data, and the slower growth rate seen in water-stressed roots, suggest that the water deficit causes a significant reduction in the rate of cell supply to the cortical cell files.We have investigated the anatomical basis for the altered growth patterns seen in response to water stress. It is well known that corn roots growing under conditions of severe water stress exhibit a much slower root extension rate than that seen in well-watered corn (6,13,17 Upon harvesting, the overall length of the roots was measured, and the apical 15 mm was excised and placed in 2.5% glutaraldehyde in 0.1 M potassium phosphate buffer (pH 7.0) overnight at room temperature. After three 15 min washes in buffer, the root apices were dehydrated in ethanol (10%, 25%, 50%, 70%; 30 min each) and stored in 70% ethanol at 4°C. The roots were dehydrated further in 95% ethanol before infiltrating. Infiltrated roots were cut and embedded as 7.5 mm segments in glycol methacrylate (Historesin, LKB-Produkter). Serial longisections were cut at 3 ,um intervals on a JB-4 microtome, and stained with toluidine blue 0 at 60°C (15). Median sections were photographed with Kodak Panatomic-X film.For transverse sections, only the apical 10 mm of the root was originally excised prior to fixation. Later, thin slices of tissue were cut 2.5 and 5.5 mm behind root apices for use in cutting transverse sections. All other procedures were the same as for the longisections.Root extension rates were calculated from initial and final root lengths over the growth period. Cell Size DataCortical cell lengths were measured on prints made from 35 mm negatives. Every 0.25 mm along the root, cell size measurements were made of the five "middle" cortical cell files on each of five to seven roots. The same five files were used for all the measurements whenever possible, but occasionally one or more of the cell measurements at a given point needed to be taken from the corresponding region on the opposite side of the stele. Cell size at each point was taken as an average of these 25 to 35 cells. Overall cell diameter was also measured at each point.
The molecular structure of trimethyl(methylene)phosphorane, (CHJJP= CH2, in the gas phase has been determined by electron diffraction. Principal distances are r,(P -C) = 181.5(3), ra(P = C) = 164.0(6) pm; and the angles between the P-C(methy1) bonds are lOl.qSY. These parameters lead to a value for the order of the highly polar P-C(methy1ene) bond of about 2.0. Die Molekiilstrulrtur von Trimethyl(methylen)phosphoran in der Gasphase aus einer ElektronenbeugungsuntersuchungDie Molekulstruktur von Trimethyl(methylen)phosphoran, (CH&P= CHI, in der Gasphase wurde durch Elektronenbeugung bestimmt. Wichtigste Atomabstande ra sind P-C = 181.5(3) und P = C = 164.0(6) pm. Die Valenzwinkel zwischen den P-C(Methy1)-Bindungen betragen 101.6(5)". Diese Parameter ergeben fur die stark polare P = C(Methy1en)-Bindung eine Bindungsordnung von etwa 2.0.
The molecular structure of hexamethylcarbodiphosphorane, (CH,),P = C = P(CH3),, in the gas phase has been determined by electron diffraction. Principle bond lengths (ra) are: P-C. 181.4(3)prn, P=C, 159.4(3) pm; C-H, 108.9(4) pm. The angles between the P-C (methyl) bonds are 101.4(3)", and the apparent P = C = P angle is 147.6(5)". The single structure that fits the experimental data closest has C2 overall symmetry, but an even closer fit is obtained if free rotation about the P = C bonds is assumed. This and other evidence indicates that the molecule is probably a symmetric top, and that the P = C = P unit is linear in the average structure, with shrinkage caused by a low frequency bending vibration giving rise to the apparent non-linearity. Bestimmung der Molekiilstruktur von Hexamethylcarbodiphosphoran in der Gasphase durch ElektronenbeugungDie Molekiilstruktur von Hexamethylcarbodiphosphoran, (CH3)3P=C= P(CH,),, wurde in der Gasphase durch Elektronenbeugung bestimmt. Die hauptsachlichen Bindungslangen (ra) sind P-C = 181.4(3)pm, P = C = 159.4(3)pm, C-H = 108.9(4)pm. Die Winkel zwischen den P-C-(Methyl)-Bindungen betragen 101.4(3)", der scheinbare P = C = P-Winkel 147.6(5)". Die einzige fixierte Struktur, die die beste Anpassung an die MeDdaten ergibt, hat die Gesamtsymmetrie C1, doch wird fur ein Model1 mit freier Drehbarkeit um die P = C-Achse eine noch bessere f h r e i nstimmung gefunden. Daraus und aus anderen Hinweisen kann geschlossen werden, da13 das Molekul einen symmetrischen Kreisel bildet und daB die P= C = P-Einheit in der gemittelten Struktur linear ist. Die beobachtete Schrumpfung geht danach auf eine sehr niedrige Beugungsfrequenz zuriick, die zur scheinbaren Nichtlinearitat fuhrt.
The title complex, [IrH(CO)(PPh,),] (1), reacts with MH,Q (M = Si or Ge when 0 = H, F, CI, Br, or I ; M = Si when Q = SiH, or CH,) to give [lrH,(CO)(PPh,),(MH,Q)]. The products have been characterised by lH, SIP, and (where appropriate) lPF n.m.r. spectra. Where M = Ge, the predominant isomer contains mutually trans phosphine ligands, whereas when M = Si the predominant (and in some cases the sole) product contains cis phosphines. In the isomers with cis phosphines, some unusual coupling patterns have been observed in the 'H n.m.r. spectra. When Q = I and M = Si, there is a difference in chemical shift between the two SiH protons of 1.3 p.p.m., and irradiation of one of the peaks at room temperature leads to the disappearance of the other, a phenomenon interpreted as due to saturation transfer; possible mechanisms for the process are discussed.
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