Philip R. Larson

Fixation patterns of 14C within developing leaves of eastern cottonwood

SummaryIndividual leaves of eastern cottonwood (Populus deltoides), representing an ontogenetic series from leaf plastochron index 0.0 to 8.0, were fed 14CO2 photosynthetically and then harvested at times ranging from 15 to 1440 min. The lamina of each fed leaf was sectioned from tip to base into 5 parts, and each part was quantitatively assayed for 14C activity. In young leaves, the percentage of the total 14C fixed (expressed in dpm/mg of dry leaf tissue) was high in the lamina tip and decreased almost linearly toward the base. With increasing leaf age, the percentage of 14C fixed decreased in the lamina tip and increased in the base. The relative activity in mature leaves was almost uniform throughout the lamina. No differences were detected in the 14C distribution patterns within leaves over the time series.On the basis of the data presented and of anatomical observations of developing cottonwood leaves, the hypothesis that the precociously mature lamina tip may provide photosynthates to the still-expanding lamina base was shown to be invalid. It is concluded that bidirectional transport in a developing cottonwood leaf results from simultaneous import to the immature basal region and export from the mature tip.

Morphology and Development of Populus deltoides Branches in Different Environments

The structure of axillary buds of Populus deltoides Bartr ex Marsh. and the development of branches from these buds were studied under conditions (1) promoting vigorous growth, (2) inducing dormancy and dormancy release, and (3) inhibiting axillary bud break. Bud growth and branch development continued without interruption on vigorous plants, but buds were inhibited on less vigorous plants growing under less favorable conditions. Bud growth declined and then ceased during dormancy induction. Both dormant and apically inhibited buds had mature protoxylem connections with the main stem; therefore, bud inhibition could not be directly attributed to lack of vascularization. Under all growth conditions, specific leaves on every branch were smaller at maturity than adjacent leaves. Anisophylly was present in primordia within the bud, and it was accentuated during leaf expansion. Small anisophyllous leaves experienced higher percentages of both leaf abortion and senescence than adjacent leaves. The presence of anisophylly throughout ontogeny, its persistence on all branches under all growth conditions, and its relation to demonstrated patterns of vascular organization, all suggest a definite role for nutritional and/or hormonal gradients.

Development and Organization of the Primary Vascular System in the Phase II Leaf and Bud of Osmunda cinnamomea L.

Cinnamon fern (Osmunda cinnamomea L.) leaf primordia of different physiological ages develop either as phase I (shoot only), as phase II (leaf + adaxial bud), or as phase III (leaf only) primordia when they are excised and cultured Third-youngest primordia (P3s) can develop either as phase II or phase III primordia in culture At the time of excision, an incipient procambial strand, which is in continuity with more mature procambium in the rhizome axis, is evident immediately beneath the leaf apical cell When a P3 develops as a phase III primordium, as it normally does in vivo, the procambium serving the leaf develops as a trace that is essentially of uniform diameter throughout its extent However, when a P3 develops as a phase II primordium, a new procambial strand diverges acropetally toward the prospective site of the adaxial bud This procambial strand is precocious; i e, it develops in advance of the bud it will serve Therefore, the adaxial bud is an induced bud A parenchymatic cleft, which forms between the acropetally developing leaf and bud procambial strands, clearly separates the leaf and bud primordia. Primary phloem differentiates acropetally and continuously, first in the leaf procambial strand and then in the bud procambial strand The timing of primary phloem differentiation in the bud trace suggests a causal relation with the appearance of an organized bud meristem Primary xylem differentiates after primary phloem It differentiates bidirectionally from an initiation site at the interstice of the parenchymatic cleft

Lamina Abortion in Terminal Bud-Scale Leaves of Populus deltoides during Dormancy Induction

Bud scales of terminal buds of Populus deltoides Bartr. ex Marsh. are formed by enlarged stipules. The laminae of the first bud-scale leaves mature and abscise, but the lamina of the last bud-scale leaf (or leaves) senesces prematurely and aborts. The immature lamina of the aborting leaf turns black, shrivels, and protrudes from the bud scales until physically dislodged during bud break the following spring. Senescence of the aborting bud-scale lamina begins with loss of starch and enlargement and vacuolation of cells abaxial to the dorsal bundle of the midrib at the lamina tip. These processes progress basipetally and laterally in the lamina, with degeneration becoming particularly evident in the spongy mesophyll. While lamina degeneration is under way, an abscission meristem begins to form distal to the stipules at the petiole base. Cell divisions initiating the abscission meristem are first evident in the reactivated adaxial meristem, and they spread laterally and obliquely across the petiole base. The vascular bundles are isolated by a funnel-shaped, protective layer of corklike cells that extends downward in the leaf base. The deteriorated central trace vasculature extends deeper in the leaf base than that of the lateral traces. We suggest that lamina senescence and abortion are in some way related to degeneration of the central leaf trace that forms the dorsal bundle of the midrib and petiole.

Structure of Leaf/Branch Gap Parenchyma and Associated Vascular Tissues in Populus deltoides

Nodes of Populus deltoides Bartr ex Marsh between leaf plastochron index 25 and 40 were examined by light and electron microscopy to determine the structure of both the birefringent (in polarized light) parenchyma (BP) occupying most of the leaf/branch gap and the spatially associated vascular tissues The BP tissue is especially well developed next to the central leaf trace and axillary branch cylinder and is notably confluent with the rays of the laterally adjacent stem xylem The BP cells have thick, lignified secondary walls with numerous pits traversed by abundant plasmodesmata Virtually all BP cells except those near vessels have diffuse cytoplasm, relatively few organelles, and often large vacuoles The stem and branch xylem consists of fibers, vessels, both vessel-associated and non-vessel-associated parenchyma cells, and isolation and contact ray cells The leaf trace xylem is made up of vessels and vessel-associated parenchyma cells Structurally intermediate cells occur where the stem xylem merges with the BP tissue Most parenchymatous cells in direct contact with vessels have dense cytoplasm and numerous organelles These may be BP cells, thin- or thick-walled vessel-associated parenchyma cells, or contact ray cells Relatively numerous plasmodesmata connect the cytoplasmically dense cells with adjacent ray, non-vessel-associated parenchyma, and BP cells Most living cells have plasmodesmata with constricted neck regions, but some BP and many ray cells also have plasmodesmata with unconstricted neck regions.

Phloem Translocation from a Leaf to Its Nodal Region and Axillary Branch in Populus deltoides

Microautoradiography was used to follow the movement of 14C through the nodal regions and in the stems of young Populus deltoides Bartr ex Marsh plants when either entire laminae were photosynthetically fed 14CO2 or cut petioles were fed 14C-labeled amino compounds The leaf/branch gap of the central (C) leaf trace is filled with both thick- and thin-walled, heavily pitted parenchymatous cells that retain their protoplasts. The C-trace translocates little 14C to mature cells of the gap region because it lacks differentiated rays at this level in the stem However, the C-trace contributes 14C to the cambium-like region that adds cells to the gap. The C-trace also transfers 14C laterally to phloem of the branch traces in the nodal region. The branch traces in turn translocate 14C according to sink demands acropetally in the branch, or basipetally in the stem, or laterally to adjacent stem traces, or inward to the gap region The 14C translocated inward via the rays is presumably deposited in the gap cells, where it accumulates as stored starch during predormancy No evidence was found that gap cells functioned as transfer cells. Little cross transfer occurred between unrelated leaf traces in the stem Thus, gap cells at nodes other than the treated node received 14C only when a labeled trace lay immediately contiguous to branch traces at that node For example, the left trace of the treated leaf contributed 14C to the branch traces and nodal region but not to the leaf trace situated three nodes below Amino compounds fed through cut petioles transported 14C primarily upward in the transpiration stream. Xylem-to-phloem transfer via the rays occurred at all levels in the stem, but it was particularly pronounced at nodal junctures No labeled amino acids were transported into the gap region

Cambial Zone Characteristics

In Chapter 4.4, several different schemes were presented for distinguishing cambial initials from their derivatives (Figs. 4.15, 4.16). According to these schemes, the cambial initials and their immediate derivatives, the xylem and phloem mother cells, were collectively referred to as the cambial zone. However, as pointed out in previous chapters, it is seldom possible to precisely identify cambial initials in histological preparations. Consequently, characteristics of the initials must, by necessity, be determined retrospectively by examining cambial derivatives in various states of differentiation.

Defining the Cambium

The terms cambial cell, cambial fiber, mother cell, and tissue mother cell were often employed in a nonspecific sense with reference to the cambium by early investigators. At times, one of these terms might be confined to the dividing cell(s), and at other times it might include derivative cells. The nonspecificity of the terms was due mainly to difficulties in isolating the cambium for microscopic observation, consequently, to difficulties in formulating a precise definition.

Anticlinal Cambial Divisions

Sachs (1878) introduced the terms anticlinal and periclinal with reference to the division planes of cells in the shoot apex. The terms were later adopted for the division patterns in cells of the lateral meristems. Anticlinal division is the process by which new cell files are added to the cambial layer (Figs. 6.1, 6.2, 6.11, 6.49, 9.6B). These divisions are sometimes referred to as pseudotransverse and multiplicative, and in the older literature as radial. Radial as opposed to tangential cell division was one of the criteria Sanio (1873) used to identify cambial initial cells in Pinus sylvestris. According to Sanio, each time a cambial initial cell divided radially, a new file of initial cells was produced that in turn gave rise to a new file of wood and bast daughter cells. The initial cell could then be located retrospectively by the doubling of a cambial initial (Fig. 6.2A). Because doubling of the radial file occurred in both wood and bast, Sanio concluded that the radial division must have taken place in the initial cell. Mischke (1890) verified this fact in Picea abies as well as in P. sylvestris by agreeing that radial divisions were confined to cambial initial cells. Hartig (1855a, 1859a, 1878), however, held fast to his view of a biseriate cambium (Chap. 4.1.1). He believed that radial increase of the cambium occurred by “diagonal segmentation” of each member of the “Muterfasernpaare”, or mother-fiber pairs (Fig. 3.5), but tangential increase occurred by “radial segmentation” of the same mother fibers. According to this scheme, a continuous double file from wood to bast across the cambium would require simultaneous radial division of both members of the mother-fiber pair, a rather unlikely event.

Cambial Cell Characteristics

The shapes of cambial cells have been a matter of concern from the time they were first recognized. Braun (1854) described cambial cells simply as elongated elements with sloping walls and pointed ends. Nageli (1858) described them similarly, but added that the pointed ends had a tendency to grow in the direction of the axis. Radlkofer (1858) considered cambial cells to be long rectangles approaching the form of a “whetstone” with oblique end walls. Most dissension among workers concerned configuration of the end wall. Muller (1875–76), for example, argued that cambial cells took the form of a pointed fiber, a view contested by Velten (1875), who claimed that Muller’s cambial cells were actually differentiating fibers.