Cell Division and Expansion and Resultant Tissue 3

By following the movement of carbon particle markers on theexposed surface of a cultured tomato apex it has been shownthat a leaf primordium is formed by growth on the flank of theapex raising the tissue upwards and outwards to form the leafbuttress. The whole of the apical surface is in an active stateof cell division and expansion except in the axillary regionabove the primordium. The data provide direct estimates of therates of division in the outer layer of cells. The distribution of blocked metaphase figures following treatmentwith colchicine, shows that in the early stages of primordiumformation cell divisions are concentrated in what appears tobo a ‘growth centre’ in the corpus to one side ofthe apical dome. As the bulge of the primordium develops, thegrowth centre spreads out and splits into two parts continuingthe growth of the dome (proximal side) and the primordium (distalside). Between these two regions of active division there arisesa small pocket of cells in the axil, whose rate of divisionrapidly declines. Cuts made in the apical surface in the early stages of primordiumformation immediately gape widely, apparently as a result ofpressure exerted on the outer layers from within by divisionsin the corpus. Once the upper surface of the primordium becomesraised above the dome, the axillary cells seem to become compressedbetween the two zones of active division. In the axil at thisstage (a) cuts do not gape but close up after exuding cell sapand (b) the carbon particle markers move slightly together.     

Flower primordium formation at the Arabidopsis shoot apex: quantitative analysis of surface geometry and growth

Geometry changes, especially surface expansion, accompanying flower primordium formation are investigated at the reproductive shoot apex of Arabidopsis with the aid of a non-invasive replica method and a 3-D reconstruction algorithm. The observed changes are characteristic enough to differentiate the early development of flower primordium in Arabidopsis into distinct stages. Primordium formation starts from the fast and anisotropic growth at the periphery of the shoot apical meristem, with the maximum extension in the meridional direction. Surprisingly, the primordium first becomes a shallow crease, and it is only later that this shape changes into a bulge. The bulge is formed from the shallow crease due to slower and less anisotropic growth than at the onset of primordium formation. It is proposed that the shallow crease is the first axil, i.e. the axil of a putative rudimentary bract subtending the flower primordium proper, while the flower primordium proper is the bulge formed at the bottom of this axil. At the adaxial side of the bulge, the second axil (a narrow and deep crease) is formed setting the boundary between the flower primordium proper and the shoot apical meristem. Surface growth, leading to the formation of the second axil, is slow and anisotropic. This is similar to the previously described growth pattern at the boundary of the leaf primordium in Anagallis.     

Changes in Poparity of Growth During Leaf Initiation in the Pea, Pisum sativum L.

In the apical dome of the pea shoot apex the axis of growthof the epidermal cells becomes predominantly longitudinal inthe I₁ region one plastochron before a leaf is initiated, andthis orientation persists into the young primordium. In contrast,in the underlying, non-epidermal cells the growth axis in theI₁ region becomes randomized half a plastochron before leafinitiation, but in the young primordium it becomes the sameas in the epidermis. The initiation of a leaf primordium thereforetakes place without any major change in the orientation of growthaxes in the epidermis. A controlling role for the epidermisis therefore suggested. No marked reorientation of the growthaxis occurs on the sides of the newly initiated primordium.The shape of the young primordium can be related to the differentialrates of growth and division within it rather than to changesin growth orientation. Pisum sativum, pea, shoot apex, meristem, growth, epidermis, polarity     

Rates of Cell Division in the Shoot Apical Meristem of Pisum

The relative rates of cell division in different regions ofthe pea shoot apical meristem were obtained by measuring theincrease in the numbers of metaphases following applicationof colchicine to the plants. Absolute values for the rates ofcell division could be calculated since the average rate ofcell division for the whole apex was known. Measurements ofthe rates of cell division were obtained at defined intervalsduring the course of a single plastochron. Within each regionof the apex the rate of cell division did not change more thanabout two-fold throughout the plastochron. There was very littleor no increase in the rate of cell division associated withleaf initiation. The formation of a leaf primordium and thesubsequent growth of the apical dome apparently result fromchanges in the direction of growth rather than changes in therates of growth. Three main regions were discernible withinthe apical meristem: a region with a slow rate of cell divisionin the apical dome, a region of a faster rate of cell divisionat the base of the apical dome and at the site of initiationof procambial strands, and a region of an intermediate rateof cell division in the newly initiated leaf primordium andthe adjacent part of the shoot axis.     

Mechanical Stress in the Shoot Apices of Euphorbia, Lycopersicon, and Pisum under controlled Turgor

Cuts were made in the surface of the shoot apices of Euphorbialathyris, tomato (Lycopersicon esculentum), and Pea (Pisum sativum)while they were completely immersed in water or aqueous mannitolat various concentrations, or in near-saturated air. Gapingoccurred all over the apical dome of Euphorbia and on the tomatoapex at the site of emergence of the primordial bulge. Maximumgaping occurred in near-saturated air and under water, and wasprogressively reduced with increasing osmotica. It is concludedthat the gaping results from tension in the surface cells andis not caused by superficial drying out. No gaping occurred in the axil of the newly formed primordiumof the tomato nor anywhere in the apex of the pea. Histologicalevidence suggests that these tissues are under lateral compression. The mechanical stresses involved are discussed in relation tothe morphology of the apices together with existing data onthe distribution of cell division during primordia formation.     

Growth of the Surface and Inner Parts of the Pea Shoot Apical Meristem during Leaf Initiation

The rate of cell division and the rate of increase in cell numberwere compared in the epidermis and in the underlying cells ofthe apical dome, the incipient primordium, and the axis of thepea shoot apex. These rates did not coincide in any part ofthe apex, but in the primordium and the apical dome there wasa closer correspondence in the epidermis than in the underlyingcells. This is interpreted as showing that the changing shapeof the apex, during growth of the primordium and the apicaldome, is associated with a tendency to local changes in therate of growth in the epidermis but to a tendency to changesin the direction of growth in the underlying cells.     

The Structure of the Plumule of Nelumbo Nucifera Gaertn. and the Nature of its Scale

The structure of the plumule of Nelumbo nucifera Gaertn. and its feature covered with scale are seldom seen in dicotyledon. The fact that the plumule possesses scale is even more uncommon. This particular phenomenon is investigated by observing the differentiation of the plumule apex and the development of the leaf organs. After the seed is formed, the embryo has two young leaves and a terminal bud covered with scale. In the bud it has already differentiated the 3rd and the 4th leaf primordium and a shoot apex, the differentiation of which is very complex. So the structure of the plumule passes through 4 plastochrons altogether. It is made clear through observation and analysis that, before the 4th leaf primordium is formed, the transforma- tions of the shoot apex of the embryo in each plastochron are fundamentally alike. After the 4th leaf primordium is developed, the shoot apex becomes complex and there appear 3 different active cell regions which become the bases of vegetative bud of the seeding apex. The development of these 3 active cell regions will be stated in “The Structure of the Vegetative Bud of Nelumbo nucifera Gaertn. and the Nature of its Scales.” The apices of the plumule are almost slightly domed in structure. As a rule, their width is from 95 to 107 μ. Their height is from 17 to 20 μ during one plastochron. Before the 3rd leaf initiation, the anatomical structure of apices is examined and the fol- lowing zones may be delimited: zone of tunica initials, zone of corpus initials, peripheral zone, and zone of rib meristems. It is frequently observed that the cell of corpus in subapical peripheral zone develops periclinal division, which is the initial cell of leaf primordium; Procambium will appear before the stage of the appearance of leaf buttress. The apex of the plumule is in an apical position, but when the seedling is formed, as the developing leaves are alternate, the directions of the shoot apex are changed, simultaneously the base part of the leaf encloses the axis, and the adaxial meristem also differentiates the scale which encloses the terminal bud, thus placing the bud in axillary of the leaf and forming a zigzag phenomenon of the axis of the seedling. Above the basal adaxial side of the leaf primordium develops the scale of the plumule with meristem periclinal division of closely attached protoderm as its base. So the scale of the plumule of Nelumbo nucifera Gaertn. and the axillary stipule are of the same origin. To sum up, the scale of the embryo of Nelumbo nucifera Gaertn. is differentiated from the adaxial meristem of the basal part of the leaf primordium, and is the derivative part of the leaf. It has the same function as the coleoptile of the monocotyledon. Whether they are homologous organs or not is still to be investigated.     

The seminal root primordia in barley and the participation of their non-meristematic cells in root construction

In addition to the primary seminal primordium, the so-called secondary seminal root primordia are also initiated in a barley embryo. The primary root primordium is developmentally most advanced. It is formed by root meristem covered with the root cap, and by a histologically determined region with completed cell division. On germination, the restoration of growth processes begins in this non-meristematic region of root primordium by cell elongation, with the exception of the zone adjacent to the scutellar node, the cells of which do not elongate but continue differentiating. In the root primordia initiated later, the zone with completed cell division is relatively shorter, in the youngest primordia the non-meristematic cells may be lacking. The root meristem is reactivated after the primary root primordium has broken through the sheath-like coleorrhiza and emerges from the caryopsis as the primary root. The character of root meristem indicates a reduced water content at the embryonic development of root primordium. With progressing growth the root apex becomes thinner, the meristematic region becomes longer, and the differences in the extent of cell division between individual cell types increase. — The primary root base is formed of cells pre-existing in the seminal root primordium. Upon desiccation of caryopsis in maturation, and subsequent quiescent period, their development was temporarily broken, proceeding with the onset of germination. The length of this postembryonically non-dividing basal zone is different in individual cell types. The column of central metaxylem characteristic of the smallest number of cell cycles, has, under the given conditions, a mean length of about 22 mm, whereas the pericycle, as the tissue with most prolonged cell division, has a mean length of about 6 mm. In the seminal root primordia initiated later the non-dividing areas are relatively shorter. The basal region of seminal roots thus differs in its ontogenesis from the increase which is formed “de novo” by the action of root meristem upon seed germination.     

The Growth of the Shoot Apex and the Apical Dome of Barley During Ear Initiation

The growth of the floral main shoot apex of spring barley wasstudied during the period of ear initiation (that is, from initiationof the collar primordium until maximum primordium number wasattained). While floral primordia were being initiated the relativelength growth rate of the shoot apex was low. After maximumprimordium number there was about a twofold increase in relativelength growth rate. Estimates of the volume, fresh and dry weightof the floral apex indicated that the relative weight growthrate was also low at first and increased after maximum primordiumnumber. The rates of growth and the size at initiation of thefloral primordia was affected by their position on the floralshoot apex. The relative volume growth rate increased acropetallyfrom the first initiated (collar) primordium. The collar wasthe smallest and each subsequently-initiated primordium increasedin length. The diameter of the newly-initiated primordium alsoincreased until more than half the primordia had been initiatedand then it declined. The apical dome increased in both lengthand diameter and both were at a maximum at the time of the double-ridgestage and then both measurements declined. Length and diameterwere at a minimum at maximum primordium number. Subsequentlythere was an increase in the length of the dome, after whichboth the dome and some of the last formed, distal primordiadied. The period of spikelet initiation therefore is a stage duringwhich the relative growth rate of the floral shoot apex is low,there are changes in the size of the dome and the primordiashow a progression of increasing relative growth rates acropetallyalong the shoot apex. These changes produce the embryo ear inwhich the most advanced spikelets are in the lower mid-partof the ear. Changes in size of the apical dome prior to maximumprimordium number may be related to the subsequent death ofspikelet primordia and therefore also to grain number in themature ear.     

In-situ localization of chalcone synthase mRNA in pea root nodule development

In this paper studies on the role of flavonoids in pea root nodule development are reported. Flavonoid synthesis was followed by localizing chalcone synthase (CHS) mRNA in infected pea roots and in root nodules. In a nodule primordium, CHS mRNA is present in all cells of the primordium. Therefore it is hypothesized that the Rhizobium Nod factor induces cell division in the root cortex by stimulating the production of flavonoids that function as auxin transport inhibitors. In nodules CHS mRNA is predominantly present in a region at the apex of the nodule consisting of meristematic and cortical cells. These cells are not infected by Rhizobium. Therefore it is postulated that CHS plays a role in nodule development rather than in a defence response. In roots CHS mRNA is located at a similar position as in nodules, suggesting that CHS has the same function in both root and nodule development. When nodules are formed by mutants of Rhizobium leguminosarum bv. viciae that are unable to secrete β(1-2) glucan and to synthesize the O-antigen containing LPS I, CHS genes are also expressed in regions of the nodule that are infected by Rhizobium. It is postulated that the impaired development of nodules formed by these mutants is due to an induction of a plant defence response.     

Planes of Cell Division and Growth in the Shoot Apex of Pisum

The planes of cell division and growth were examined in thecourse of a single plastochron in the shoot apical meristemby observing the orientations of mitotic spindles. In the I₁region of the apical dome, cell divisions were at first anticlinalbut 30 h before a leaf primordium emerged at this site 20 percent of the cell divisions became periclinal. These periclinaldivisions were found only in the corpus. Periclinal divisionsin the tunica were coincident with the appearance of the primordiumas a bulge. The change in the direction of growth in I₁ at thesite of the incipient leaf primordium occurred without any changein the rate of growth in this region of the meristem.     

马尾松雌球果的发生和早期发育研究

采用常规石蜡制片技术对马尾松雌球果的发生和早期发育进行了研究。结果表明：雌球果原基发生时间为10月中旬,不同的树龄和着生部位,其发生时间不同。雌球果原基与营养茎端在外部形态及内部细胞组织学分区结构有明显差异。营养茎端外形扁平,内部顶端分生组织结构有顶端原始细胞区、中央母细胞区、形成层状过渡区、周围分生组织区及肋状分生组织区5个明显的分区;而雌球果原基外形呈圆锥状,内部结构只有套层和髓区。12月初,最初的苞片原基在雌球果原基的鳞片的叶腋处产生,之后其由基部向顶部连续发生。翌年1月初,在苞片原基的叶腋处,珠鳞原基发生,发生方向亦为向顶发育。2月底,苞片体积不再发生变化,珠鳞膨大端的基部的近轴面分化出2个倒生胚珠。从雌球果原基发生到胚珠分化历时4个多月。亚热带的冬季气候对马尾松雌球果的生长发育没有明显的抑制作用。     

Changes in Volume and Cell Number in the Different Regions of the Shoot Apex of Pisum during a Single Plastochron

The length of the ninth plastochron in shoot apices of Pisumsativum was measured and found to be almost 46 h. This singleplastochron was divided into 11 morphologically recognizablestages and the time taken to reach each stage was measured.The cell number and cell volume of five regions of the apexwas measured at each stage of the plastochron. Although theapex as a whole grew exponentially, growth during the first30 h of the plastochron was predominantly in the primordiumand the adjacent tissues, whereas in the last 16 h growth wasmainly in the apical dome. Since the mean cell volume remainedconstant, different rates of growth were due to different ratesof cell division. The data suggested that the apex probablygrows by the formation of growth centres on alternate sidesof the apex, the beginning of each new growth centre being apparentas an increased rate of growth in the apical dome 16 h beforethe beginning of the next plastochron. The inception of a newprimordium may therefore precede its appearance as a bump byabout 16 h, and precede the first periclinal division in thetunica by 26 h. A central zone of larger cells with lightly-stainingnuclei was found at the extreme apex. This central zone becamereduced in size or disappeared at the time at which a new primordiumwas about to become visible.     

Biophysical mechanisms for morphogenetic progressions at the shoot apex.

Leaf primordia, first visible as small bumps, are produced in a cyclical pattern at the edges of the shoot apex, a smooth region at the top of the stem. Their formation is a biomechanical process. This review first considers hypothetical construction mechanisms and then summarizes research that provides information about how and where the primordia are made. Studies of growth at the primordium site indicate the importance of growth parallel to the surface in generating the forces for primordium emergence. The symmetry of the pattern of reinforcement by cellulose microfibrils correlates with the subsequent pattern of primordium production. Finite element models of the apex reveal that lateral bulging of the apex results in a gradient of shear stress, with high shear at the future primordium site. In contrast, tension parallel to the surface is lowest at the primordium site. Response of apical surface tissue to punctures indicates that an existing primordium can exert a pulling force tangential to its base and a compressive force perpendicular to its base. These observations lead to identification of a continuous biophysical cycle for apex morphogenesis, in which most of the steps are direct physical consequences of the previous step. Biophysical processes, subject to input from genetic, hormonal, and environmental sources, are thus involved in the construction and patterning of leaf primordia.     

Stomatogenesis and Formation of Cirri in Fragments of Oxytricha fallax Stein

Of fragments involving a partial removal of the original adoral zone of membranelles (AZM), the monomacro-nucleate ones become reorganized monostomes resulting from a simple fusion of the remnant AZM to the oral primordium induced, and the binucleate ones become dividers by initiation of the oral primordium posteriorly from the posterior terminal of the remnant AZM. The cirral primordium in any fragment arises alongside its corresponding oral area. Weisz's idea of the dominance and inhibition of the original oral system extending over the oral primordium site is applicable in stomatogenesis of the present species. This application is found also in cirral formation.
In fragments from early stage dividers, a formed oral primordium is easily absorbed by influence of the intact original AZM. This event also occurs after complete removal of the AZM. Such results led to the hypothesis that the oral primordium in the normal divider may be formed under some stoma-togenic activation of the AZM followed by escape from inhibition also arising from the same source. Irrevocable furrow formation and irreversibility of the oral primordium in stomatogenesis occur in later stages of division. Nevertheless division in these stages is blocked when certain operations are performed, forming monsters possessing the AZM of the opisthe translocated to the side opposite to that of the proter. In other monsters obtained from a fusion of the AZM of the proter to that of the opisthe, division occurs belatedly, prior to which secondary oral and cirral primordia are produced.     

慈姑匍匐茎的起源和球茎的膨大研究

匍匐茎的发生一般见于主茎倒二或倒三叶原基的叶腋部位。在匍匐茎发生区域的主茎一侧,匍匐茎原始细胞的基部形成壳状区;壳状区的形成对匍匐茎原基的外凸起一定作用。匍匐茎无居间分生组织;它的伸长依靠顶端分生组织细胞的横向分裂,使轴向细胞数目增多,并使细胞的轴向延伸。球茎的膨大是通过匍匐茎第8—10节基本分生组织的细胞有丝分裂,增加细胞数目,然后细胞体积的扩大来实现的。球茎中的淀粉一般为单粒淀粉;匍匐茎中的淀粉由单粒和复合两种淀粉粒组成。     

黄鳞伞子实体发育

为了丰富大型真菌个体发育学研究数据,并依据个体发育学数据提出分类学建议,通过石蜡切片法对黄鳞伞子实体进行了发育学观察。结果显示:黄鳞伞子实体发育初期原基圆形至卵圆形,具外菌幕原基,内部菌丝弯曲致密,螺旋交织在一起呈束状,随着原基的发育,端向膨大生长,菌盖原基出现,与此同时原基基部在保持伸长生长外,直径也明显增大,菌柄原基由此形成。随后菌褶腔出现,栅栏状细胞的出现表明子实层发育的开始,发育过程中Y型菌褶清晰可见。黄鳞伞的发育属于菌盖菌柄型发育,发育类型是半被果双菌幕发育型。     

FLORAL DEVELOPMENT OF COCHLOSPERMUM TINCTORIUM AND BIXA ORELLANA WITH SPECIAL EMPHASIS ON THE ANDROECIUM

The initiation and development of the flower of Bixa orellana L. and Cochlospermum tinctorium A. Rich, were investigated using the scanning electron microscope to elucidate the nature of the androecial development and the relationships of both taxa. Initiation of floral buds starts with the inception of five sepals in a 2/5 sequence between two bracteoles. The petals are formed successively on the irregular pentagonal apex in a phyllotactic pattern different from that of the calyx. Stamen development proceeds centrifugally on a broad circular primordium or ringwall, which attains its final size at the time of initiation of individual stamens. Stamen primordia arise in successive whorls without connection to the perianth. The residue of the apex is differentiated into a gynoecial circular primordium. In Cochlospermum 3 to 4 carpels are initiated; in Bixa no individual carpels are visible. The origin of the androecial circular primordium is discussed in relation to other types of androecial development. A comparison is made with existing theories of the evolution of multistaminate androecia. Few differences were found in the floral development of Bixa and Cochlospermum, except in the gynoecium. It is proposed to retain them in a single family Bixaceae for a number of reasons. The floral development supports a thealean, dillenialean, or violalean affinity rather than a malvalean. The inception of a broad androecial circular primordium is highly different from the development in Malvales, where more space for stamens is provided by the continuous growth of a tube.