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.     

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.     

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.     

LOCALIZATION OF CELL DIVISION ACTIVITY IN THE PRIMARY THICKENING MERISTEM IN ALLIUM CEPA L

Stems 1, 2, 3 months old of Allium cepa L. were labelled with tritiated thymidine, fixed in FAA, sectioned, stained with the Feulgen reaction, and prepared for autoradiography. The serial transverse sections were outlined with a camera lucida, recording labelled nuclei as dots. These drawings were used for 3-dimensional reconstructions of the locations of labelled nuclei. Near the top of the stem, labelled nuclei occur in a broad band, whereas they occur in narrower bands at successively lower levels in the stem, and finally labelled nuclei disappear. The locations of the labelled nuclei correspond to the location of the primary thickening meristem (PTM) in the stem of onion as determined by previous histological and histochemical observations. Microspectrophotometry was used to measure the relative amounts of DNA in Feulgen-stained nuclei of the PTM in serial transverse sections of 1- and 2-month-old onion stems. A bimodal distribution was obtained which can be explained by changes in DNA levels during the cell cycle. No evidence of polyploid nuclei was observed. One can conclude, therefore, that the PTM is the site of cell division activity during the primary stem thickening process in onion.     

HISTOCHEMISTRY OF THE PRIMARY THICKENING MERISTEM IN THE VEGETATIVE STEM OF ALLIUM CEPA L.

Stems of Allium cepa L., 1, 2, 5, and 6 months old respond similarly when stained for protein and RNA. The primary thickening meristem (PTM) stains more intensely than surrounding stem tissues. The acropetal region of the PTM is a broadly staining band which narrows basipetally to the level of the initiation of shoot-borne roots in the stem and disappears more basipetally. These staining patterns are consistent with the hypothesis that the PTM functions in stem thickening and root production, and also indicate that the meristem functions before histological evidence of the cambial-like zone exists in the onion stem. Histochemical staining may be an accurate method of locating the PTM.     

ONTOGENY AND CORRELATIVE RELATIONSHIPS OF THE PRIMARY THICKENING MERISTEM IN FOUR-O'CLOCK PLANTS (NYCTAGINACEAE) MAINTAINED UNDER LONG AND SHORT PHOTOPERIODS

The developmental anatomy of Mirabilis jalapa was investigated during the first 90 days of growth. The primary thickening meristem (PTM) initially differentiates in the pericycle at the top of the cotyledonary node 18 days after germination, then basipetally in the pericycle through the hypocotyl. The PTM differentiates acropetally into the stem and in the pericycle of the primaiy root, commencing 22 days after germination. Endodermis is easily identifiable in hypocotyls as well as in primary roots because of Casparian thickenings in its cells. It has not been definitely identified in stems. There are three rings of primary vascular bundles in the stem. The PTM differentiates as segments of cambium in a layer of cells (probably in the pericycle) on an arc between vascular bundles of the outer bundle ring. Later, arcs of PTM differentiate externally to the phloem of each bundle. Each arc forms a connection between original segments of PTM lying on either side of each vascular bundle. Thus, the PTM becomes a continuous cylinder. The PTM differentiates in the pericycle outside vascular tissue in the hypocotyl and root. Differentiation of the PTM and the mode of secondary thickening is similar in plants exposed to short (8-hr) and to long (18-hr) photoperiods, but some differences were observed. The PTM differentiates closer to the stem apex in all plants over 18 clays of age growing vegetatively under long photoperiods. That is, the diffuse lateral meristem, in whose cells the PTM differentiates in young intemodes, is shorter in nearly all investigated plants growing in long photoperiods. The hypocotyl and base of the primary root of 40-day-old plants in short photoperiods were more enlarged than those of the same age plants in long photoperiods; but, at the end of 64 days, the hypocotyl and primaiy root base were larger in plants growing under short photoperiods. Thirty-four days after seed germination, flower initiation occurs in plants exposed to short photoperiods. One hundred fifty days after seed germination, flowers differentiate on plants exposed to long photoperiods.     

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.     

ON THE CORRELATION OF PRIMARY ROOT LENGTH,MERISTEM SIZE AND PROTOXYLEM TRACHEARY ELEMENT POSITION IN PEA SEEDLINGS

Pea seedlings (Pisum sativum L. cv. Alaska) were darkgrown in vermiculite. Roots of various lengths were cleared, stained and measured to determine the relative meristem height (MH), width and volume and the distance to the most proximal protoxylem tracheary element (PTE). A correlation was found between root length, MH and PTE position as follows: in roots from 4–40 mm as the root elongated the MH lengthened and PTE position increased its distance from the body/cap juncture; in roots 40–80 mm MH and PTE position remained approximately constant; in longer roots (80–120 mm) MH became shorter, and PTE position closer to the tip as the root elongated. The relationship, using our measurement procedure was that for every 0.19 mm change in MH, the PTE position changed by 1 mm. This response was partially growth rate dependent since short roots (4–80 mm) grew at a constant rate and longer roots (80–140 mm) grew slower. Root manipulations and trifluralin treatment, to inhibit cell division, caused tip swelling and modulated the position of the PTE toward the root tip. The control of the spatial relationship between meristem size and maturation position is discussed.     

THE OCCURRENCE OF INTERCALARY AND UNINTERRUPTED MERISTEMS IN THE INTERNODES OF TROPICAL MONOCOTYLEDONS

The distribution of percent of dividing nuclei, parenchyma cell length, total cell number per internode, and total internode length were determined for successive internodes in the apex and growing vegetative internodes of 23 tropical species in 17 families of monocotyledons. Basal intercalary meristems (IM) were found in representatives of Commelinaceae, Cyperaceae, Flagellariaceae, Poaceae, Restionaceae, and Marantaceae. Uninterrupted meristems (UM) which are confined progressively to the upper region of the internode and are not isolated meristematic regions were found in the Costaceae, Dioscoreaceae, Philesiaceae, Smilacaceae, Agavaceae, Araceae, Arecaceae, Liliaceae, Pandanaceae, and Zingiberaceae. Both IM and UM were found in different species of Orchidaceae. The only morphological trait correlated with meristem type was presence of sheathing leaf bases in all species with IM. Both IM and UM are interpreted as extensions of the primary elongating meristem; the IM is disjunct, and the UM is continuous with it. The phytomer growth unit and the presence of internodal IM's cannot be applied generally to the monocotyledons.     

THE LATERAL MERISTEM AND ITS DERIVATIVES IN THE CORM OF ISOETES MURICATA

Corm tissue of Isoetes muricata Dur. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. Very young secondary sieve elements can be distinguished from contiguous cambial cells by their distinctive plastids and by the presence of crystalline and/or fibrillar proteinaceous material in dilated cisternae of rough endoplasmic reticulum (ER). At maturity, the sieve elements are lined by the plasmalemma and a parietal, anastomosing network of smooth ER. Degenerate nuclei persist in all mature sieve elements. In addition, mature sieve elments contain plastids and mitochondria. Sieve-area pores are present in all walls. The lateral meristem of I. muricata consists of 2–3 layers of cells year-round. Judging from numerous collections made between October 1972 and July 1975, new sieve-element differentiation precedes cambial activity by about a month. Early in May, 1–2 cells immediately adjacent to already mature sieve elements differentiate directly into sieve elements without prior division. In early June, at about the time sieve-element differentiation is completed, cambial division begins. Division is sporadic, not uniform throughout the meristem. Dormancy callose accumulates in the secondary sieve elements in late October, and is removed in early May, at about the same time new sieve-element differentiation begins. Cells of the dormant cambium are characterized by the presence of numerous small vacuoles and large quantities of storage materials, including lipid droplets, starch grains, and tannin. By contrast, active cambial cells contain few large vacuoles with little or no tannin, and they have little storage material.     

PHYLOGENETIC ANALYSIS OF THE MALE ONTOGENETIC PROGRAM IN AQUATIC AND TERRESTRIAL MONOCOTYLEDONS

The male program of ontogeny in flowering plants encompasses the events from meiosis of microsporocytes to fertilization. Three main sequences are discussed; the deposition of cell walls, changes in cytoplasmic organelles, and the program of nuclear divisions leading to the formation of two sperm cells and a vegetative cell in each pollen grain. Variations in these ontogenetic sequences are particularly apparent in the monocotyledons, which exhibit diversity in pollen morphology, wall structure, and mode of pollination. The male program of development has been compared in selected terrestrial monocotyledons belonging to the Liliaceae and Gramineae and aquatic members of the Cymodoceaceae, Najadaceae, and Zannichelliaceae. A total of 26 characters from the male program are discussed and then used to construct a cladogram derived only from developmental data for the five species. The polarity of only a few of the character transformations has been determined directly by observation of developmental sequences; most have been interpreted by outgroup analysis.     

THE SYNTHESES OF DEOXYRIBONUCLEIC ACID AND HISTONE IN THE ONION ROOT MERISTEM

A comparison of the times necessary to incorporate tritium-labeled lysine and arginine into histones and tritium-labeled thymidine into DNA indicates that the periods of DNA and histone synthesis prior to division closely coincide. (The comparison was made by determining the times necessary, after pulse labeling, for cells with marked chromosomes to enter and then leave the division stages.) An additional period of chromosomal protein synthesis, of short duration, occurs late in interphase. Most of the chromosomal proteins appear either to be synthesized in the nucleus or to migrate there shortly after synthesis. Much of this protein is conserved from one division to the next. Studies of the effects of puromycin and fluorodeoxyuridine on the syntheses of DNA and histone suggest that continuation of DNA synthesis is dependent on a concurrent protein synthesis. Histone synthesis, on the other hand, can proceed at a normal rate under conditions in which DNA synthesis is inhibited.