THE ONTOGENY OF THE PRIMARY THICKENING MERISTEM OF ATRIPLEX HORTENSIS L. (CHENOPODIACEAE)

Seedlings of Atriplex hortensis were studied to ascertain; 1) in which organ the primary thickening meristem (PTM) first differentiates; 2) the direction of differentiation of the PTM, and 3) the pattern of differentiation of conjunctive tissue. The PTM initially differentiates in pericycle of the primary root base 11 days after emergence of the primary root. It then differentiates in the transition region of the hypocotyl, mostly in cells of pericycle between pairs of vascular bundles. In the upper hypocotyl, PTM differentiates by day 20 in the inner layer of cortical parenchyma. In the epicotyl, PTM apparently differentiates in the inner layer of cortex, by day 24. Desmogic xylem differentiates from radial files of internal conjunctive tissue cells and desmogic phloem differentiates opposite desmogic xylem strands from newly formed cells of external conjunctive tissue. No interfascicular cambium differentiates in the root, hypocotyl, or epicotyl.     

ONTOGENY OF THE PRIMARY THICKENING MERISTEM IN SEEDLINGS OF BOUGAINVILLEA SPECTABILIS

Anomalous secondary thickening occurs in the main axis of Bougainvillea spectabilis as a result of a primary thickening meristem which differentiates in pericycle. The primary thickening meristem first appears in the base of the primary root about 6 days after germination and differentiates acropetally as the root elongates. It begins differentiating from the base of the hypocotyl toward the shoot apex about 33 days after germination. The primary thickening meristem is first observable at the base of the first internode about 60 days after germination. It then becomes a cylinder in the main axis of the seedling. No stelar cambial cylinder forms in the primary root, hypocotyl, or stem because vascular cambium differentiation occurs neither in the pericycle opposite xylem points in the primary root nor in interfascicular parenchyma in the hypocotyl or stem. The primary vascular system of the stem appears anomalous because an inner and an outer ring of vascular bundles differentiate in the stele. Bundles of the inner ring anastomose in internodes, whereas those of the outer ring do not. Desmogen strands each of which is composed of phloem, xylem with both tracheids and vessels, and a desmogic cambium, differentiate from prodesmogen strands in conjunctive tissue. The parenchymatous cells surrounding desmogen strands then differentiate into elongated simple-pitted fibers and thick-walled fusiform cells that are about the same length as the primary thickening meristem initials.     

ANOMALOUS SECONDARY THICKENING IN PHYTOLACCA AMERICANA L. (PHYTOLACCACEAE)

Differentiation of the primary thickening meristem (PTM) was investigated in seedlings and older plants of Phytolacca americana L. Initiation of the PTM occurs in pericycle or inner cortex at the hypocotyl-primary root junction of young plants. Differentiation of the PTM in stems occurs acropetally in a cylinder of randomly dividing cells termed the diffuse lateral meristem (DLM). The PTM produces secondary tissue to the inside (internal conjunctive tissue) and to the outside (external conjunctive tissue). Patches of xylem and phloem differentiate, opposite each other, in recently produced internal and external conjunctive tissue, respectively. The resulting strands (desmogen strands) of xylem and phloem are secondary in origin, and are peripheral to primary vascular tissues. Phloem of desmogen strands usually differentiates first. Xylem of desmogen strands is composed of both tracheids and vessel elements; the latter sometimes becoming occluded with tyloses and unidentified substances. As root and hypocotyl increase in diameter, cylinders of PTMs differentiate successively and centrifugally in external conjunctive tissue. Even though the first PTM differentiates in pericycle or inner cortex and later PTMs differentiate in external conjunctive tissue, all are referred to as PTMs because of their similar activity. Multiple rings of desmogen strands can be observed in transections of lateral roots, primary roots and hypocotyls. Throughout the length of the stem, only one ring of desmogen strands is present. Fewer rings of desmogen strands are present in the top of the hypocotyl and cotylendonary node, as compared to the subjacent hypocotyl, due to anastomoses of centrifugally differentiating desmogen strands.     

MORPHOLOGY OF HUMULUS LUPULUS. II. SECONDARY GROWTH IN THE ROOT AND SEEDLING VASCULARIZATION

Miller , Robert H. (U. Nevada, Reno.) Morphology of Humulus luppulus. II. Secondary growth in the root and seedling vascularization. Amer. Jour. Bot. 46(4): 269–277. Illus. 1959.—In the primary state the roots of Humulus lupulus L. have a diarch xylem plate with 2 strands of primary phloem lying on either side of the primary xylem. Secondary histogenesis is described for the primary root. Fibrous and fleshy storage roots are developed by the hop plant and their respective developmental and anatomical structures are described. Lateral roots are initiated in the pericycle opposite the protoxylem poles. The architecture of these secondary roots is similar to that of the primary root. The seedling develops a fleshy storage organ through secondary growth of the primary root and the hypocotyl. The hypocotyl eventually resembles a fleshy taproot throughout most of its extent. The vascular cambium differentiates large amounts of parenchymatous tissues. A relatively smaller amount of tracheary tissue is formed. The secondary phloem comprises a high percentage of phloem parenchyma and ray cells containing numerous large starch grains, and constitutes the larger portion of the fleshy storage root. Numerous thick-walled lignified fibers occur throughout the secondary vascular tissues. Resin and tannin cells are abundantly distributed. A phellogen is differentiated from the pericycle and develops a persistent periderm on the outer surface of the fleshy storage organ. A relatively short transition region occurs in the upper part of the hypocotyl. The transition takes place from a radially alternate arrangement of the vascular tissues in the root to a collateral arrangement in the cotyledons.     

Organ identity of the thalloid plant body of Griffithella hookeriana and Polypleurum stylosum – Podostemoideae (Podostemaceae)

The morphological nature of the thalloid plant body of podostemads has remained controversial for long. The present investigation was carried out on two members of the Podostemoideae i.e. Griffithella hookeriana and Polypleurum stylosum to understand their organ identity. The origin of the plant body was traced from the embryo by germinating the seeds under aseptic conditions. Mature embryo of both species does not show an identifiable shoot apical meristem (SAM) and root apical meristem (RAM). Upon germination, the radicular pole does not form a primary root but differentiates adhesive hairs. At the cotyledonary junction, SAM is initiated that differentiates 6–9 leaves apically (primary axis) and a primordium laterally. This primordium subsequently emerges from the hypocotyl and develops into a thalloid plant. The latter has been interpreted as a flattened stem because it not only shows tunica-corpus like organization at the tip but also originates endogenously from the same SAM that forms the `primary axis'.     

RADIAL GROWTH IN THE CYCADALES

The development of radial growth which leads to the pachycaulous form was investigated in eight of the 10 genera of the Cycadales; i.e., Ceratozamia, Cycas, Dioon, Encephalartos, Macrozamia, Microcycas, Stangeria, and Zamia. In all taxa, development of radial growth is essentially the same: a primary thickening meristem is differentiated in the stelar region of the cotyledonary node of the seedling at germination and produces derivatives mainly centrifugally. This primary thickening meristem (PTM) then differentiates acropetally and becomes continuous with the peripheral zone of the shoot apex. At first the PTM is a vertical cylinder, but as the seedling continues to grow into an adult plant, the PTM shows a more horizontal orientation (like an open umbrella) and produces the broad cortex. Secondary growth is by a vascular cambium which produces secondary xylem to the inside and secondary phloem to the outside. The broad pith originates from derivatives of the rib meristem of the massive shoot apex. The seedling and young plant is composed of a shortened shoot (i.e., no internodes) produced by the PTM and rib meristem, and a large fleshy primary root which results from a diffuse growth pattern. Individual cells in both the pith and cortex of the root divide. Their derivatives divide at right angles to the original division plane. Thus, quartets and even octets of cells are recognizable and can be traced to individual parent cells.     

金盏菊幼苗形态解剖学研究

金盏菊种子萌发过程中凡下胚轴生长较快、伸长较长的个体均形成壮苗。下胚轴大部分区段同时具有根、茎初生结构的特征。过渡区位于下胚轴上部。子叶迹与上胚轴维管组织雏形在子叶节区发生分离。     

Anatomy of Jojoba (Simmondsia chinensis) seed and the utilization of liquid wax during germination

Much work has been done on the agricultural potential of Jojoba, but little on the anatomy of the mature plant or seed. Our investigations concern the structure of the embryo of mature seeds and their external morphology during early germination. The embryo is straight and investing. A hypocotyl sheath surrounds the radicle like a hollow cone. The apical meristem is a low mound of cells in a shallow depression between the broad short petioles of the cotyledons. During germination these petioles lengthen and force the embryo away from the coytledons and seed coat. The hypocotyl elongates and the primary root rapidly extends and is well developed before the apical meristem becomes active. A mature imbibed seed contains approximately fifty percent liquid wax. After germination there is a linear decrease in the amount of wax to approximately ten percent at thirty days.     

THE RELATIONSHIP BETWEEN THE PRIMARY THICKENING MERISTEM AND THE SECONDARY THICKENING MERISTEM IN YUCCA WHIPPLEI TORR. I. HISTOLOGY OF THE MATURE VEGETATIVE STEM

Anatomical observations were made on 1-, 2-, and 3-yr-old plants of Yucca whipplei Torr, ssp. percursa Haines grown from seed collected from a single parent in Refugio Canyon, Santa Barbara, California. The primary body of the vegetative stem consists of cortex and central cylinder with a central pith. Parenchyma cells in the ground tissue are arranged in anticlinal cell files continuous from beneath the leaf bases, through the cortex and central cylinder to the pith. Individual vascular bundles in the primary body have a collateral arrangement of xylem and phloem. The parenchyma cells of the ground tissue of the secondary body are also arranged in files continuous with those of the primary parenchyma. Secondary vascular bundles have an amphivasal arrangement and an undulating path with frequent anastomoses. Primary and secondary vascular bundles are longitudinally continuous. The primary thickening meristem (PTM) is longitudinally continuous with the secondary thickening meristem (STM). Axillary buds initiated during primary growth were observed in the leaf axils. The STM becomes more active prior to and during root initiation. Layers of secondary vascular bundles are associated with root formation.     

ANATOMY OF SEEDLING ROOTS OF PSEUDOTSUGA MENZIESII

The seedling root system of Pseudotsuga menziesii (Mirb.) Franco consists of the primary root, active long laterals, long laterals that become mycorrhizal, and short roots that may or may not become mycorrhizal. Numerous adventitious roots arise from the pericycle in young roots and from the vascular cambium and pericycle in older roots following pruning. All actively growing apices have a single plate of initials, a complex zonation of mother cells, and a similar pattern of primary tissue differentiation. Short roots and mycorrhizal short roots have 2 plates of initials, one producing the stele and the other the root cap and cortex, and differentiation occurs close to the apex. Primary and adventitious roots are usually triarch, while long laterals are usually diarch as are all short roots. The latter lack secondary xylem, but mycorrhizal short roots may produce a small amount of secondary phloem.     

DEVELOPMENTAL ANATOMY IN ROOTS OF IPOMOEA PURPUREA. II. INITIATION AND DEVELOPMENT OF SECONDARY ROOTS

In seedlings of Ipomoea purpurea secondary roots are initiated in the primary root pericycle opposite immature protoxylem. Cells derived from immature endodermis, pericycle, and incipient protoxylem and stelar parenchyma contribute to the primordium. The derivatives of the endodermis become a uniseriate covering over the tip and flanks of the primordium and emerged secondary root; the endodermal covering is sloughed off when the lateral root reaches 1–5 mm in length. A series of periclinal and anticlinal divisions in the pericycle and its derivatives gives rise to the main body of the secondary root. The initials for the vascular cylinder, cortex, and rootcap-epidermis complex are established very early during primordium enlargement. After emergence from the primary root, the cortical initials undergo significant structural modifications related to enlargement of the ground meristem and cortex, and the rootcapepidermal initials are partitioned into columellar initials and lateral rootcapepidermal initials. Procambium diameter increases by periclinal divisions in peripheral sectors. The mature vascular cylinder is comprised of several vascular patterns, ranging from diarch to pentarch, that are probably related ontogenetically. Cells derived from incipient protoxylem and stelar parenchyma cells of the primary root form the vascuar connection between primary and secondary roots.     

THE RELATIONSHIP BETWEEN THE PRIMARY THICKENING MERISTEM AND THE SECONDARY THICKENING MERISTEM IN YUCCA WHIPPLEI TORR. III. OBSERVATIONS FROM HISTOCHEMISTRY AND AUTORADIOGRAPHY

Observations were made of stem sections stained for RNA and protein of Yucca whipplei ranging from germinated seedlings to 6-month-old plants. One-, two-, and three-month-old plants were labeled with tritiated thymidine, fixed in FAA, sectioned, stained with the Feulgen reaction, and prepared for autoradiography. The serial transverse sections were outlined with a drawing tube recording all labeled nuclei on a computer graphics tablet. Computer-assisted three-dimensional reconstructions were made to observe the locations of labeled nuclei. The two techniques are in agreement: the thickening meristem is broad near the top of the stem, occupies a narrower band at more basipetal levels, and disappears below the level of recent root initiation. There are no gaps in staining or labeling, and there are no changes in staining or labeling that would distinguish between the activities of the primary thickening meristem and the secondary thickening meristem in those plants which possess both. The meristems are continuous at all stages of development in the young vegetative stem. The STM is interpreted to be a developmental continuation of the PTM.     

白鲜根的发育解剖学研究

应用半薄切片、常规石蜡切片并结合离析法,对药用植物白鲜(Dictamnus dasycarpus Turcz.)根的发生发育过程进行了研究。结果表明:白鲜根的发生发育过程包括4个阶段,即原分生组织阶段、初生分生组织阶段、初生结构阶段以及次生结构阶段。原分生组织位于根冠内侧及初生分生组织之间,衍生细胞分化为初生分生组织。初生分生组织由原表皮、基本分生组织以及中柱原组成。原表皮分化为表皮,基本分生组织分化为皮层,中柱原分化为维管柱,共同组成根的初生结构;在初生结构中,部分表皮细胞外壁向外延伸形成根毛,皮层中分布有油细胞,内皮层有凯氏带,初生木质部为二原型或偶见三原型,外始式;根初生结构有髓或无。次生结构来源于原形成层起源的维管形成层的活动以及中柱鞘起源的木栓形成层的活动;白鲜次生韧皮部宽广,其中多年生根中可占根横切面积的85%,另外除基本组成分子外,还分布有油细胞;周皮发达,木栓层厚;初生皮层、次生木质部和次生韧皮部薄壁细胞中常充满丰富的淀粉粒。     

Lateral meristems and stem thickening growth in monocotyledons

Although monocotyledons lack a vascular cambium of the type found in dicotyledons and conifers, lateral meristems still play an important role in the establishment of their growth habits. The presence near the shoot apex of a primary thickening meristem (PTM), which is probably plesiomorphic in monocotyledons, predisposes evolution into the many pachycaul forms. A PTM occurs in virtually all monocotyledons, whereas the secondary thickening meristem (STM), which is morphologically similar, is limited to a few genera of Liliiflorae. these records are reviewed in a systematic context. To a greater or lesser extent in different taxa, the PTM is responsible for primary stem thickening, adventitious root production, and formation of linkages between stem, root and leaf vasculature. The STM largely contributes to the body of the stem. The sometimes obscure distinction between the two meristems, and their relationship with other stem meristems are discussed. For systematic purposes stem thickening in monocotyledons is separated into two characters: diffuse growth (as in palms), and growth by means of lateral meristems. The three states of the second character are represented by the first three of Mangin’s (1882) four categories (two herbaceous, the third arborescent): (1) The lateral meristem is limited in extent, and ceases activity after root formation. (2) It remains active for a limited period after cessation of root formation, contributing to the plant body. (3) It remains active throughout the life of the plant, contributing the bulk of the plant body.     

双子叶植物出土幼苗根茎转变区维管组织发育动态

关于根茎初生维管系统之间的连接以及与子叶的关系,在文献中已有广泛的论述,有过各种不同的解释。大部分早期关于根茎转变区的文献研究的是初生组织已完成发育的幼苗。这些研究者认为转变区域是根和茎这两种轴器官之间维管组织发生转变、相互连接的区域。但由于茎中的初生维管组织可以认为是叶迹及叶迹的延伸的综合,转变区域应被看作是轴维管系统与叶迹维管系统之间的连接。因此,转变区的研究必须说明根维管系统与最早的真叶叶迹之间的关系。通过对北乌头和大豆胚胎及幼苗维管组织的解剖学研究,本工作显示在出土萌发的双子叶植物中,初生维管组织在根-下胚轴-子叶中形成一连续系统,并完成根与子叶叶迹之间的维管组织过渡转变。而上胚轴中的维管组织是位于根-下胚轴-子叶上方独立形成的第二维管系统。上胚轴中维管组织的分化起始于第一真叶叶迹基部,向上分化进入叶片,向下进入胚轴并在子叶节下方与根-下胚轴-子叶维管系统相连接。真叶叶迹的木质部与下胚轴中靠近韧皮部的后生木质部或次生木质部连接。根与上胚轴之间不存在维管组织的过渡、转变,而只是在同样发育方向的组织中有一种直接的简单的连接．     

THE PRIMARY THICKENING MERISTEM: DEFINITION AND FUNCTION IN MONOCOTYLEDONS

The PTM should be defined as a diffuse primary meristem which decreases in cross-sectional extent (i.e., becomes a thinner-walled cylinder) in a basipetal direction. It is associated with extensive anticlinal cell files and consists of cell initials that divide predominantly in periclinal planes. This meristem occurs typically in monocotyledons, especially those with thick, compact stems in species with rosette shoot axes. The PTM is also associated with a wide crown, so that the apical meristem is either slightly above the level of youngest leaf primordia, at approximately the same level as the leaf primordia, or distinctly sunken below surrounding stem tissue and the youngest leaf primordia. The location is dependent on the extent of primary thickening growth occurring in a particular species. A meristem associated with primary thickening of other plant groups should not be called a primary thickening meristem unless all of the above characteristics are shown to be associated with the meristem being examined. The primary thickening meristem is responsible for primary thickening of a stem axis. Its ontogenetic relationship with the STM needs further investigation. Extensive primary stem thickening has been observed in non-monocotyledons (ferns, lycopods, cycads, and dictyledons). Some of these organisms appear to undergo primary thickening from a PTM in a similar process as that which occurs in monocotyledons. Further research is necessary to establish the mechanisms of primary thickening in these cases.     

Pericycle cell proliferation and lateral root initiation in Arabidopsis

In contrast with other cells generated by the root apical meristem in Arabidopsis, pericycle cells adjacent to the protoxylem poles of the vascular cylinder continue to cycle without interruption during passage through the elongation and differentiation zones. However, only some of the dividing pericycle cells are committed to the asymmetric, formative divisions that give rise to lateral root primordia (LRPs). This was demonstrated by direct observation and mapping of mitotic figures, cell-length measurements, and the histochemical analysis of a cyclin-GUS fusion protein in pericycle cells. The estimated duration of a pericycle cell cycle in the root apical meristem was similar to the interval between cell displacement from the meristem and the initiation of LRP formation. Developmentally controlled LRP initiation occurs early, 3 to 8 mm from the root tip. Thus the first growth control point in lateral root formation is defined by the initiation of primordia in stochastic patterns by cells passing through the elongation and young differentiation zones, up to where lateral roots begin to emerge from the primary root. Therefore, the first growth control point is not restricted to a narrow developmental window. We propose that late LRP initiation is developmentally unrelated to the root apical meristem and is operated by a second growth control point that can be activated by environmental cues. The observation that pericycle cells divide and lateral root primordia form without intervening mitotic quiescence suggests that lateral organ formation in roots and shoots might not be as fundamentally different as previously thought.     

Reduced de-etiolation of hypocotyl growth in a tomato mutant is associated with hypersensitivity to, and high endogenous levels of, abscisic acid.

A recessive single gene mutant, 7B-1, in tomato was originally selected for its photoperiod-dependent male sterility. The 7B-1 mutant also has some pleiotropic effects including reduced light-induced inhibition, i.e. de-etiolation, of the hypocotyl in long days (LD), increased seed size and weight, and reduced transpiration rate. These traits led us to investigate the sensitivity of 7B-1 to exogenous hormones and the interaction of these responses with daylength. In LD, but not in short days (SD), 7B-1 was more sensitive than wild-type (WT) to exogenous abscisic acid (ABA) for inhibition of seed germination, root elongation and transpiration rate. 7B-1 mutant also exhibited reduced responses to exogenous gibberellin (GA(3)) for hypocotyl elongation, and to inhibitors of GA biosynthesis for seed germination and root and hypocotyl elongation. 7B-1 hypocotyls contained a higher level of endogenous ABA than WT in both photoperiods, although ABA levels were higher in LD than in SD. In contrast, growth-active GAs, i.e. GA(1), GA(3) and GA(4), and IAA were low in the mutant hypocotyls. The 7B-1 mutant appears to be an ABA-overproducer, and the photoperiod-regulated ABA levels may be responsible for the hypersensitivity of the mutant to exogenous ABA.     

银州柴胡根的发育解剖学研究

本研究采用常规石蜡切片结合荧光显微镜技术对银州柴胡根的发育解剖学进行了研究。结果表明:（1）银州柴胡根顶端分生组织由原分生组织及其衍生的初生分生组织组成。原生分生组织细胞体积小、排列紧密、细胞质浓厚、细胞核大而明显,具有典型的分生组织的特点;（2）初生分生组织由根冠原、表皮原、皮层原和中柱原组成。在根发育过程中,表皮、皮层和维管柱共同组成其初生结构。银州柴胡根初生木质部为二原型或三原型,外始式;同时在根表皮细胞的径向壁观察到径向壁的细胞壁加厚;（3）在根次生生长过程中,位于初生木质部和初生韧皮部之间的原形成层恢复分裂能力产生维管形成层,维管形成层不断地向外产生次生韧皮部,向内产生次生木质部;同时位于根内皮层内方的中柱鞘细胞恢复分裂能力产生木栓形成层,木栓形成层向外形成木栓层,向内形成栓内层。在维管形成层和木栓形成层分裂的过程中,在次生韧皮部和中柱鞘组织中产生形态大小不同的分泌道,均为次生的裂生型分泌道。研究认为,银州柴胡根的结构类似于药典收录的北柴胡和红柴胡根的结构特点,但其根表皮细胞径向壁加厚、木纤维的分布、分泌道的大小和数量等有别于柴胡属其它植物,可作为柴胡属植物重要的分类鉴定依据。     

PHOTOPERIODISM IN AMARANTHUS CAUDATUS. I. A RE-EXAMINATION OF THE PHOTOPERIODIC RESPONSE

Zabka , George G. (State U. Iowa, Iowa City.) Photoperiodism in Amaranthus caudatus. I. A re-examination of the photoperiodic response. Amer. Jour. Bot. 48(1): 21–28. Illus. 1961.—Under the conditions described in this study, Amaranthus caudatus is not subject to inductive short days until it has reached its “sensitive period” or age which is approximately 30 days after the time of germination. Beyond this sensitive period, 2 days are sufficient to initiate inflorescence primordia. Macroscopic identification of this response is possible 2–3 days later, if the plants are retained on short photoperiods. Continued development of the inflorescence is also promoted by short days. This species will also initiate inflorescence primordia on long days of 18 hours duration approximately 60 days beyond germination. Consequently, this is not an obligate short-day plant as previously described. Although A. caudatus will initiate primordia on long days, subsequent normal development of the inflorescence proceeds only under short photoperiods. Plants initiating primordia on long or short photoperiods and then placed on long photoperiods will produce inflorescences which are stubby, generally recurved and spread at the apices. Subsequent flowering and seeding is also delayed. Plants initiating primordia on long days and then placed on short days develop mature inflorescences rapidly but they are divided at the apices and exhibit numerous basal branches.