Dormancy in Seeds of Charlock: II. THE INFLUENCE OF THE SEED COAT

An attempt was made to elucidate those factors associated withthe seed coat which are responsible for maintaining the dormancyof the charlock seed. The rates of water uptake by the seedand by the embryo were measured; the permeability of the coveringlayers to water was much the same as that of parenchymatouscells. The action of certain dormancy-breaking treatments, viz.embryo excision, exposure to concentrated sulphuric acid, alternatingtemperatures, and gibberellic acid, were therefore examined.Fromexperiments with excised embryos and seeds exposed to concentratedsulphuric acid, it appeared that the loss of dormancy followingthe removal of the seed coat resulted from an increase in oxygensupply to the embryo. The germination of a small proportionof seeds at low temperature may also be due to an increase inthe oxygen concentration within the seed. The dormancy-breakingeffect of gibberellic acid, however, is unlikely to be due tochanges in the permeability of the seed coat to oxygen. Estimatesof the rate of oxygen uptake and growth of seeds treated withgibberellic acid at various external oxygen concentrations showthat the resistance of the covering layers to diffusion of oxygenretards, but does not prevent, the initiation of growth.It wouldappear that dormancy is maintained in charlock seeds by theaction of a specific growth-inhibiting substance which is producedat low oxygen concentration in the interior of the seed andwhich diffuses to the meristems.     

Dormancy in Seeds of Charlock: IV. INTERRELATIONSHIPS OF GROWTH, OXYGEN SUPPLY AND CONCENTRATION OF INHIBITOR

Respiration measurements were made over a period of 24 h at25 °C on seeds and excised embryos maintained in Warburgflasks with partial pressures of oxygen ranging from 0 to 1atm. In the initial phase (0 to 4 h), the rate of oxygen uptake(Q_O2 of excised embryos increased linearly with external oxygenconcentration (C_O from 0 to 0.1 atm O₂ from 0.1 to 0.2 atm O₂the relation was curvilinear, and from 0.2 to 1.0 atm O₂ uptakewas independent of concentration. In later stages the relationbetween Q_O2 and C_o changed, and from 20 to 24 h the rate ofoxygen uptake increased with concentration to 1.0 atm O₂. Thechanges with time were associated with increase in rate of respiration,increase in cell size and cell number, and the oxidation offats. The decline in concentration of oxygen from the surfaceto the centre of embryos was calculated to be relatively smallat each external oxygen concentration. Althugh the rate of diffusionfailed to keep pace with consumption, the main parameters whichdetermined the internal oxygen status of the embryos were thesurface concentrations and the permeability of the seed coat.The resistance of the seed coat to diffusion of oxygen was foundto be very high, the coefficient of diffusion being about 10^–7mm² s^–1. The concentration of oxygen and in air were estimatedto be approximately 0.04 and 0.02 atm O₂, respectively. Sincea smaller concentration of oxygen (0.012 atm O₂) in the tissueswas found to be sufficient for growth, the dormancy of the seedswas not due to lack of oxygen. Dormancy appeared to be due tothe activity of growth-inhibiting substances, the concentrationof which increased with decrease in oxygen supply; below 0.1atm O₂ their rate of production increased with decrease in theoxygen concentration of the tissues. They accumulated withinthe testas of dormant seeds and prevented cell elongation. Extractsof the inhibitory substances were partially purifield by partitioningthe aqueous fraction with ether and separating chromatographically.The active principle(s) was not abscisic acid ((+)—AbscisinII, ‘Dormin’) or the mustard oil, allylisothiocyanate.     

Dormancy in Seeds of Charlock: III. OCCURRENCE AND MODE OF ACTION OF AN INHIBITOR ASSOCIATED WITH DORMANCY

The dormancy of charlock seeds appears to be associated withthe presence of an inhibitor which accumulates within the seed.This inhibitor diffuses into the external solution when seedsare placed in water under germination conditions or controlledexperimental conditions. A small quantity of inhibitor diffusesfrom the seed coats but most arises from the embryo. The increasein temperature was measured by comparing the relative growth-ratesof the radicles of excised charlock embryos in water and intest solutions of diffusate. The results suggest that the rateof accumulation of inhibitor in the external solution is controlledby diffusion, and that there is continuous production of inhibitorin the tissues of the embryo which have a low oxgyen tension.The critical concentration of the inhibitor which completelyprevents cell elongation is rapidly attained in seeds whichare dormant. The inhibitor is unlikely to be a mustard oil,such as allylisothio-cyanate.     

ANATOMY OF THE SEED OF CONVOLVULUS ARVENSIS

Sripleng , Aksorn , (Kasetsart U., Bangkok, Thailand), and Frank H. Smith . Anatomy of the seed of Convolvulus arvensis. Amer. Jour. Bot. 47(5) : 386—392. Illus. 1960.–The anatropous ovule has a small, ephemeral nucellus covered by a massive integument. Shortly after fertilization, a lateral pouch develops from the upper portion of the embryo sac toward the dorsal side of the ovule and then downward. This leaves a partial integumentary septum in the base of the seed. The cellular endosperm is mostly absorbed by the embryo. Two—6 cell layers persist on all sides of the seed except below the cotyledons on the dorsal side where larger amounts persist. Over most of the seed the dermatogen develops into an epidermis that consists in part of groups of thick-walled elongate cells that produce the papillose appearance of the mature seed. The cells beneath the dermatogen divide periclinally and form 2 layers. The outer layer undergoes anticlinal divisions and differentiates a subepidermal layer of small, rectangular, thick-walled cells that become lightly lignified and suberized. The cells of the inner layer undergo some anticinal and periclinal divisions, elongate and differentiate as palisade sclerenchyma. The inner layers of the integument consist of parenchyma cells that are crushed and partially absorbed at maturity. The pad on the basal end of the seed, between the hilum and micropyle, is derived from a multiple epidermis that is differentiated into several layers of rectangular cells and a layer of palisade sclerenchyma. The subepidermal and palisade layers found over other parts of the seed dip beneath the pad.     

THE DEVELOPMENTAL ANATOMY OF OPUNTIA BASILARIS. I. EMBRYO,ROOT, AND TRANSITION ZONE

The large seeds of Opuntia basilaris Engelm. & Bigel. show an unusually high percentage of germination, followed by a rapid development of the seedling during the first 30 days of growth. The primary root has six xylem arms alternating with six phloem poles around a large central pith. Development of metaxylem opposite each of the primary phloem poles results in the formation of eight collateral bundles. Secondary and tertiary roots have four xylem and phloem poles with xylem developing to the center of the stele. The transition zone is characterized by a gradual disappearance of all but two of the primary xylem arms of the root. Metaxylem development in the central portion of the transition zone interconnects the protoxylem poles forming a primary xylem cylinder around the central pith. The collateral bundles pass through the transition zone essentially without change.     

DEVELOPMENTAL ANATOMY OF SECRETORY CAVITIES IN THE MICROSPOROPHYLLS OF GINKGO BILOBA L.

Two to five secretory cavities develop in the hump region of the microsporophylls of Ginkgo biloba. A developing cavity is first recognized as a spherical pocket of large, densely cytoplasmic cells (central secretory cells) in the median portion of a microsporophyll primordium. These cells degenerate and a small cavity is formed which is filled with the contents of the degenerating cells. Flattened incurved cells (parietal secretory cells) develop around the disintegrating central secretory cells and slough off into the enlarging cavity. Thus, the cavities develop by lysigeny. A mature cavity is surrounded by senescent parietal secretory cells, scanty parenchyma, and a loosely fitting epidermis. Histochemical tests indicate the presence of lipid and pectic substances in the cavities. Previous reports on the morphological interpretation and possible function role of the cavities are discussed in the light of the present investigation.     

DEVELOPMENTAL ANATOMY IN ROOTS OF IPOMOEA PURPUREA. I. RADICLE AND PRIMARY ROOT

The developmental anatomy of the primary root of Ipomoea purpurea was studied at several growth stages, beginning with the radicle. The radicle is generally composed of three superimposed tiers of initials, which produce the vascular cylinder, cortex, and columella; and a peripheral band of lateral rootcap-epidermal initials. The radicular cortex contains 16–19 immature laticifers; none of the tissue regions in the radicle contains mature cells. Following germination and during the first 2–3 cm growth of the primary root the apical meristem and its derivative tissues undergo a series of modifications. Root apical diameter decreases as cells in lateral portions of the rootcap elongate; meanwhile, the columella enlarges vertically. The relationship between cortical and columellar initials changes as fewer mitoses occur in the former while the latter remain active. In longer roots the columellar initials are directly in contact with the vascular initials. Cortical size diminishes during early root growth as cortical laticifers and their associated cells cease to be produced by the outer cortical initials and ground meristem. Early procambium, at the level of vascular pattern initiation, decreases in diameter by cellular reorientation, and the vascular cylinder decreases in overall diameter although the tetrarch pattern remains unchanged.     

DEVELOPMENTAL ANATOMY OF CORALLOID ROOTS IN CYCADS

Coralloid roots of cycads were found to originate endogenously from the pericycle of apogeotropic secondary roots or adventitious roots that have become exposed or nearly exposed to the soil surface. All mature coralloid roots are susceptible to infection by algal endophytes, which seem to enter from the soil through a break in the dermal layers. In the coralloid roots the algae inhabit intercellular spaces in a definite zone that arises from the protoderm, and in which the cells elongate radially following algal infection. The zone is completely surrounded by a persistent rootcap which is interpreted by most authors as a secondary cortex. The secondary cortex was shown to be derived from the rootcap in this investigation.     

Dormancy of Rice Seed: I. THE DISTRIBUTION OF DORMANCY PERIODS

The mean dormancy period is defined as the mean time taken fromharvest for the individual seeds of a population to attain theability to germinate under normal conditions. Evidence is presentedwhich shows that the distribution of dormancy periods amongthe individual seeds of a pure line is normal. Since germinationusually reaches 100 per cent., and because a normal distributionis symmetrical, a good estimate of the mean dormancy periodcan be obtained by observing the time taken from harvest tothe point when 50 per cent. of the population is capable ofgermination as shown by a series of germination tests. The mainerror involved is due to the fact that there is no obvious wayof deciding, to within a few days, when grain is ripe for harvest.Harvesting the seed prematurely tends to speed up the processesleading to loss of dormancy whereas late harvesting has littleinfluence on the date on which the mean dormancy period is achieved;it follows that when estimating the mean dormancy period theerror is much smaller if seed is harvested too early ratherthan too late. Contrary to some previous reports, no correlation has been detectedbetween ‘duration’ (period from sowing to modalflowering plus 30 days) and mean dormancy period among differentvarieties.     

THE DEVELOPMENT OF THE SEED OF ABUTILON THEOPHRASTI I. OVULE AND EMBRYO

Winter , Dorothy M. (Iowa State U., Ames.) The development of the seed of Abutilon theophrasti. I. Ovule and embryo. Amer. Jour. Bot. 47(1): 8–14. Illus. 1960.—Abutilon theophrasti Medic, is a widespread annual weed which produces an abundance of seed in capsules which mature within 20 days after pollination. Ovule differentiation may be observed at least 8 days before anthesis when a sporogenous cell becomes evident and 2 integuments are initiated. An 8-nucleate embryo sac is produced from the chalazal megaspore approximately 2 days before anthesis. The outer integument of the mature campylotropous ovule consists of 2 cell layers, the inner integument has 6 to 15 cell layers. The initially free-nucleate endosperm becomes cellular betwen 3 and 7 days after pollination. At maturity a thin layer of gelatinous endosperm encases the embryo. The Asterad-type proembryo of Abutilon has a stout suspensor and develops rapidly. Four days after pollination cotyledons are initiated; 4 days later a leaf primordium is evident. Fifteen days after pollination the embryo, which has essentially completed its growth, consists of a large hypocotyl with root promeristem and root cap at its basal end, and 2 flat, folded, leaflike cotyledons enclosing a small epicotyl at its upper end. The epicotyl consists of an embryonic leaf and a stem apex.     

ONTOGENY IN MONOCOTYLEDONS AS REVEALED BY STUDIES OF THE DEVELOPMENTAL ANATOMY OF PERICLINAL CHLOROPLAST CHIMERAS

The developmental anatomy of apically stable periclinal chloroplast chimeras was studied in a number of monocotyledonous genera. Their ontogeny is basically similar to that of dicotyledons. In both there are three independent apical layers (L-I, L-II, and L-III) whose derivatives can be traced in stem, leaf and flower. The bulk of the stem tissue is derived from L-III with only the epidermis and one or two hypodermal cell layers from L-I and L-II. All three layers participate in formation of the leaf with great flexibility in the amount of tissue from each. There is more instability in growth of most monocotyledonous leaves than in dicotyledonous leaves. As a result there is relatively more tissue derived from L-I and L-II and less from L-III. The same is true in floral development so that a significant number of gametes are of L-I origin. As in dicotyledons, there is variation in direction, timing, and frequency of cell division. However, an overriding genetic control results in normal size, shape, and structure. The evidence from genetic and cytochimeras in both monocotyledons and dicotyledons has provided direct experimental proof of the functional reality of the apical layers described by Hanstein and Schmidt.     

THE DEVELOPMENT OF THE SEED OF ABUTILON THEOPHRASTI. II. SEED COAT

Winter , Dorothy M. (Iowa State U., Ames.) The development of the seed of Abutilon theophrasti. II. Seat coat. Amer. Jour. Bot. 47(3) : 157—162. Illus. 1960.–The integuments of Abutilon theophrasti Medic. undergo a rapid increase in size, predominantly by anticlinal cell divisions during the first 3 days after fertilization. Within 7 days, the outer epidermis of the inner integument becomes thick walled. At maturity this compact, lignified, and cutinized palisade layer accounts for more than half the thickness of the seed coat. During early growth, the palisade cells form a continuous layer in the micropylar region. In the chalazal region the palisade layer is discontinuous in a slit-shaped region, 60 × 740 microns. The shape of this discontinuity constitutes a major difference between dormant-seeded Abutilon and non-dormant Gossypium seeds. Exterior to the palisade layer is the outer integument which consists of a small-celled layer and a large-celled layer sparsely covered with unicellular, lignified hairs. Interior to the palisade is the thick mesophyll of the inner integument which is largely digested during seed growth and leaves only 2 pigmented cell layers in most regions at maturity. The inner epidermis is small-celled, pigmented and cutinized and adheres tightly to the endosperm. Seed coat impermeability increases with seed maturity. Even immature seeds will germinate, if scarified, indicating a lack of embryo dormancy.