AN INVESTIGATION OF THE PHYLOGENETIC AND ONTOGENETIC VARIABILITY OF SHOOT APICAL MERISTEMS IN THE CACTACEAE

The sizes, shapes and zonations of the shoot apical meristems of 22 species of cacti were examined. This family was chosen because of its great diversity of habits; the more primitive members are nonsucculent. leafy trees and more advanced members are highly specialized “leaf-less” stem-succulents. By combining these measurements with those already in the literature, a sample of almost 70 species was obtained. Apical meristems range in size from only 80 μm in diam in some species to as much as 1.500 μm in diam in others. The shape ranges from being flat to almost hemispherical. Despite the great range in size and shape of the apical meristems, or the range in the morphologies of the leaves and stems which are produced by the meristems. all apices had the usual zonation: tunica, central mother cells, peripheral zone, and pith-rib meristem. The sizes of each of the zones. expressed either as the number of cells per zone or expressed as a percentage of the whole apex. were highly variable. The variation in apical dimensions and zone sizes occurred both phylogenetically and ontogenetically. and this has been interpreted to indicate that the morphogenetic mechanisms which control apical size and zonation are easily modified, both during the development of individual plants and during the evolution of new species.     

香榧营养苗端细胞组织分区的超微结构观察

本文研究了榧树(Torreya grandis)成熟植株在季节生长中营养苗端的超微结构变化。各区域细胞的主要区别特征为:顶端原始细胞与亚顶端细胞相接的细胞壁较厚,液泡多分布于细胞游离面,质体中淀粉粒较小;亚顶端细胞壁较厚,液泡较大,质体中淀粉粒较大而多;周缘区细胞质体多不具淀粉粒,液泡也较小,胞间连丝丰富;肋状区细胞被大量的含淀粉质体及液泡占据了大部分空间,胞间连丝丰富。在季节变化的四个时期中,各区域细胞的亚显微结构特征亦不相同。休眠期各区细胞淀粉质体较发达,细胞壁较厚,液泡大;叶扩展期淀粉质体减少或消失;芽鳞形成期出现大量小液泡;新的顶芽形成期液泡增加,核糖体含量较高。讨论了各区域细胞核形态与其细胞活跃性的关系。     

AN ELECTRON MICROSCOPIC EVALUATION OF CYTOHISTOLOGICAL ZONATION IN THE SHOOT APICAL MERISTEM OF PINUS BANKSIANA

Shoot apical meristems of jack pine (Pinus banksiana) were examined by light and electron microscopy. Cytohistological zonation was evident when meristems were fixed in Craf IV, embedded in paraffin, and stained with Chlorazol Black E. When meristems were fixed for electron microscopy the cytoplasm of the apical initials and central mother cells each contained numerous lipid bodies and their nuclei contained little, if any, heterochromatin. The cytoplasm of the peripheral zone was rich in ribosomes. The nuclei of the peripheral zone and rib meristem were heterochromatic. Thus, the lack of heterochromatin in the nuclei and the dissolution of lipids in the cytoplasm of the apical initials and central mother cells appeared to contribute most to the organization and appearance (cytohistological zonation) of the shoot apex when standard histological techniques are used.     

IN VITRO DEVELOPMENT OF THE ISOLATED SHOOT APICAL MERISTEM OF ANGIOSPERMS

Development of complete plants was achieved from isolated shoot apical meristems of Nicotiana tabacum L., Daucus carota L., Nicotiana glauca Grah., Tropaeolum majus L., and Coleus blumei Benth. The explants consisted of only meristematic dome tissue with no visible leaf primordia. A simple nutrient medium composed of the Murashige and Skoog salt mixture, 100 mg/liter myo-inositol, 0.4 mg/liter thiamin-HCl, 1-2 mg/liter IAA, 30 g/liter sucrose, and 1% agar was adequate. Histologically there occurred principally tissue enlargement during the first 3-6 days, followed by appearance of bipolar organization in 6-9 days and formation of a well-defined root apex and initiation of first leaf primordium by 12 days.     

SIMULATIONS OF CELL DIMENSIONS IN SHOOT APICAL MERISTEMS: IMPLICATIONS CONCERNING ZONATE APICES

Surface areas, differential curvatures, volumes, and dimensions of cell-like units were described for various geometric shapes approaching the morphology of shoot apical meristems (SAM) in vascular plants. Geometric relationships are given in both graphic and dynamic simulations of shape. If the surface of the SAM is described by the revolution of a parabolic, hyperbolic, or circular function, then the greatest change in the surface areas or volumes of equally spaced transverse sections will occur in those regions showing the greatest differential in surface curvature. Subdivision of the SAM volume into cell-like units leads to quantitative expressions of changes in “cell” length to width and width to depth (aspect) ratios. Dependent upon the geometry of the SAM, differential expressions of aspect ratios may lead to a zonate pattern within the SAM corresponding to a central-mother-cell zone (CMC), a peripheral zone (PZ), and a pith-rib meristem (PRM). The boundary between the PRM and PZ, as seen in median longitudinal section, is a geometric consequence of the deployment of cells with aspect ratios best suited to occupy the entire SAM volume. The number of cell-like lineages, L, in the SAM may be expressed by the aspect ratio of cells in longitudinal section, n_x-y, such that for the PRM and PZ, L = k₁n_x-y^-5.38 and L = k₂n_x-y^3.18, respectively. Cellular patterns seen in the “corpus” of the SAM may not, therefore, be the a priori result of physiologically distinct populations of cells. Computer simulations are discussed within the data set derived from a study of the SAM of cacti.     

DEVELOPMENT OF VACUOLES AND LIPID BODIES IN APICAL MERISTEMS OF PINUS BANKSIANA

Shoot apical meristems of jack pine were examined weekly during the first 8 weeks post-germination with light and electron microscopy. Most of the storage lipids were utilized by the end of the 2nd week. A few lipid bodies, possibly high in phospholipid content, remained in the apical initials and central mother cells and, during the 3rd week, gave rise to vacuoles via lamellar or myelin-like structures which were first seen on their periphery. The inter-lamellar spaces enlarged and eventually a vacuole was formed. At 5 weeks, elongate and spherical osmiophilic inclusions, presumably lipid, were found in the endoplasmic reticulum. Lipid bodies, visible with light microscopy, began to accumulate in the apical initials and central mother cells in the 6th week.     

SHOOT APICAL MERISTEMS AND MUTATION: FIXATION OF SELECTIVELY NEUTRAL CELL GENOTYPES

Shoot apical meristems are interpreted as either structured, that is having a permanent set of apical initials, or stochastic, having apical initials which represent “... momentary representatives of the continuous meristematic residue at the apex of the relevant layer or zone” (Newman, 1965). The two main parameters of stochastic growth are the average number of apical initials (α) and the number of mitotic cycles (r) of the initials and their daughter cells prior to the random selection of subsequent initials. Mathematical analysis and computer simulation studies of stochastic growth have shown that if one starts with 1 mutant initial and α-1 nonmutant initials, eventually a mosaic plant results. The frequency of shoot apices composed of mutant cells is 1/α and the frequency of shoot apices composed of only nonmutant cells is (1 – α)/α. These asymptotics are only attained after considerable growth, thus mericlinal chimeras can persist for many nodes and give the appearance that a permanent set of initials is present.     

芸薹属植物发育过程中顶端结构的解剖学研究

本文采用解剖学方法研究花椰菜、青花菜、结球甘蓝和大白菜在生长发育过程中顶端分生组织结构的变化及之间存在的差异。结果显示它们的顶端分生组织结构都是由最初幼苗的原套-原体结构逐渐发育到过渡型分区结构、典型化五个分区结构,至开始进入生殖生长时期的四个分区结构(形成层状细胞区消失)。四种植物在进入生殖生长后,顶端分生组织细胞行为不同:大白菜和甘蓝顶端亚外套两侧细胞分裂分化形成顶生叶原基,在顶生叶原基内侧的细胞将进行分裂产生花序侧枝原基。花椰菜和青花菜顶端亚外套两侧细胞分裂形成花序分生组织,花序分生组织增生即为花球体;内部解剖结构表现为分生组织不断分裂增多的过程。这些结果为研究花序表型发生的解剖学本质及分子生物学研究分生组织发育方向奠定了基础。     

CHANGES IN GLYCERALDEHYDE 3-PHOSPHATE DEHYDROGENASE ACTIVITY IN SHOOT APICAL MERISTEMS OF BRASSICA CAMPESTRIS DURING TRANSITION TO FLOWERING

An assay system has been developed for the histochemical determination of glyceraldehyde 3-phosphate dehydrogenase (G3PD) activity to indicate glycolytic pathway capacity during evocation in the shoot apical meristem of Brassica campestris L. G3PD activity was differentially distributed in a zonate pattern within the meristems at the vegetative, the transition, and the floral stages. The activity of G3PD changed in all apical zones of evoked apices, but especially in the central and the peripheral zones of apices at the prefloral stage. In the prefloral stage of development heavy enzyme activity was localized in island-like areas within the peripheral zone. These results indicate that 1) the capacity of glycolysis fluctuates during evocation, 2) during floral evocation the capacity of the glycolytic pathway parallels the capacity of the citric acid cycle and the electron transport system only at the prefloral stage, and 3) G3PD activity marks incipient floral primordia. It is proposed that the enzymic marking of an incipient floral primordium indicates the end of evocation in Brassica.     

除草剂拿草特对菟丝子幼苗茎尖分生区有丝分裂的影响

酰胺类除草剂拿草特对菟丝子幼苗生长具有强烈的抑制作用。采用石蜡切片法和细胞扫描分光光度法,通过测定对照及处理幼苗茎尖分生区的细胞分裂指数和核DNA含量,发现拿草特对菟丝子幼苗生长的抑制是由于它对分生区细胞有丝分裂的双重抑制作用:间期抑制和中期抑制。即它既可阻止有丝分裂的启动,又可象秋水仙素一样通过影响纺锤丝的功能而将细胞分裂阻止于中期,引起分生区细胞的多倍体化和径向膨大。这种生长受到抑制且茎尖粗肿的幼苗不能够在农作物上寄生。     

THE SHOOT APICAL ONTOGENY OF THE PICEA ABIES SEEDLING. I. ANATOMY,APICAL DOME DIAMETER,AND PLASTOCHRON DURATION

Developing embryos in immature Picea abies seeds already have well-delineated shoot apical meristems with clearly evident cytohistological zonation. During early seedling development the zonation characteristic of gymnospermous apical meristems is attained. Seedling development is also accompanied by an approximately threefold increase in apical dome diameter. The latter approaches a steady state about 140 days after germination. Seedlings display a spiral phyllotaxis consisting of a contact parastichy system, usually of the primary Fibonacci series. As the seedlings age and apical domes enlarge, higher Fibonacci number-pairs characterize their phyllotaxis. Mathematical analysis of the relation between cumulative leaf number and age revealed that the length of the plastochronic time interval declines from about 18.5 hr to 5.7 hr as seedling age increases from 20 to 140 days.     

THE SHOOT APICAL ONTOGENY OF THE PICEA ABIES SEEDLING. IV. PROTOXYLEM INITIATION AND AGE OF INTERNODES

In Picea abies seedlings the distance below the base of the shoot apical dome to the first protoxylem (px) to be differentiated in the internodes beneath is a linear function of apical dome basal diameter. By using mathematical relations derived in earlier papers of this series, we computed the morphogenic age in plastochrons and the chronometric age in days of the internode in which px is first differentiated (n_px). As the seedlings age from 30 to 140 days, the distance from the base of the apical dome to px increases from 186 to 295 μm, the n of n_px increases from 16 to 53, but the chronometric age of n_px remains within the range of 10 to 12 days. Protoxylem differentiation in young internodes is, therefore, more closely related to chronometrie age than it is to morphogenic age or to distance from the base of the shoot apical dome.     

A MORPHOMETRIC STUDY OF THE ULTRASTRUCTURE OF ECHINOCEREUS ENGELMANNII (CACTACEAE). I. SHOOT APICAL MERISTEMS AT GERMINATION

At germination the shoot apical meristems of Echinocereus engelmannii were discs with a volume of 666,000 μm³ and were composed of a unistratose tunica (volume: 260,000 μm³) and a corpus which was two cell-layers thick (volume: 406,000 μm³). Four days after germination the nucleus constituted 28.9% of the volume of the cell, and the vacuole constituted 24.5%. The mitochondria were 13.3% of the volume of the tunica cytoplasm, the chloroplasts 9.4%, and the dictyosomes only 1.2%. The endoplasmic reticulum was too sparse to be accurately measured. The organelles of the corpus were identical in size and shape to those of the tunica, but there were statistically significant differences in their cellular and cytoplasmic densities: the more distal corpus layer (C1) was less vacuolate (16.6% of the cell volume), and both corpus layers contained more chloroplasts, 12.0% of the cytoplasmic volume in C1 and 14.3% in the more proximal corpus layer (C2). During the first four days after germination there was a dramatic increase in the size of the central vacuole (e.g., from 15.4% to 24.5% in the tunica), and the mitochondria increased in density from 10.2% of the cytoplasmic volume to 13.3%. Chloroplast density also increased in all meristem layers, but the dictyosome density decreased, as much as a 30% loss in C2. There was also a highly significant reduction in the number of cisternae per dictyosome, from 5.47 to 4.77. Surprisingly, there was no change in heterochromatin: ca. 27% of the volume of the nuclei of all layers was heterochromatic at all stages studied. Thus, the organellar structure of corpus cells is distinctly different from that of tunica cells, and as the apical meristem becomes active after germination, the changes which occur are not uniform in the meristem but rather are zone-specific.     

PRIMORDIAL LEAF AND PHYTOHORMONE EFFECTS ON EXCISED SHOOT APICAL MERISTEMS OF COLEUS BLUMEI BENTH.

Coleus blumei Benth. apical meristems and apical meristems +1, +2, +3 primordial leaf pairs were cultured to examine phytohormone influences on development and correlative effects of developing primordial leaves on in vitro responses. The meristem with no phytohormones or low levels of IAA could not develop in vitro. At least 0.1 mg/l IAA and optimumly 1-2 mg/l IAA were required for development into complete plants. IAA from 0.1 to 3 mg/l also resulted in root development with no apparent leaf or shoot formation. Levels of IAA higher than 3 mg/l were inhibitory to development. Kinetin, as a substitute for naturally occurring cytokinins, alone (0.0003 to 3 mg/l) resulted in development of rosettes of leaves. In the presence of IAA (***1 mg/l) and kinetin (0.003 mg/l) plants, rosettes, individual leaves with roots, and roots developed from isolated meristems. Glutamine and adenine sulfate both appeared inhibitory to meristem development. With +1, +2, +3 developing primordial leaf pairs left attached to the apical dome, three pairs were required for plant formation in the absence of phytohormones. In the presence of IAA, two pairs of primordial leaves resulted in plant formation; whereas, with IAA and low levels of kinetin one pair of primordial leaves was enough. Higher levels of kinetin were inhibitory to plant development with primordial leaves present. ABA appeared to be inhibitory to development of meristems and meristems +1, +3 primordial leaves at low concentrations and resulted in death at ***1 mg/l. Developing primordial leaves appear to supply the apical meristem with a balance of phytohormones during growth. Meristem development into a plant first involved formation of leaf primordia. Establishment of a bipolar axis with root formation followed.     

CONSTANCY OF RELATIVE VOLUMES OF ZONES IN SHOOT APICAL MERISTEMS IN CACTACEAE: IMPLICATIONS CONCERNING MERISTEM SIZE,SHAPE, AND METABOLISM

The volumes and dimensions of shoot apical meristem zones were determined for 21 species in 10 genera of the Cactaceae. If the volumes of the central-mother-cell zone (CMCZ), the peripheral zone (PZ), and the pith-rib meristem (PRM) are expressed as percentages of the total volume of the corpus, then they are remarkably constant (CMCZ = 11.1% of the volume of the corpus, PZ = 61.2% and PRM = 27.1%) regardless of the relative size of the apical meristems. The relative volume of the tunica decreases, however, as the whole apex increases. The zones have variable shapes, and whereas the PZ and PRM are always trapezoidal in longitudinal section, in apices with large volumes, these zones have a flatter, more flaring shape than they have in smaller apices. Thus large apices are flatter and less hemispherical than are small apices. The CMCZ, however, maintains a constant shape and is always relatively hemispherical regardless of its volume. A hypothesis that relates all of these shape and volume relationships as an integrated growth sequence is proposed.