Abstracts of Papers - Harvard Forest - Harvard University

infected with vascular pathogens appear adjacent to
vein endings. Terminating vessel members in leaves
may be sites of accumulation of pathogen metabolites,
degraded cell wall components or wound reactants.
Anatomical studies of these terminal vessel members
has provided scant evidence for their importance in
vascular wilt disease and has centered on phylogenetic
comparisons of terminating vessels. Our study there-
fore was aimed at a detailed anatomical appraisal of
terminating vessels especially the structure of end
walls. Leaves of tomato, chrysanthemum, alfalfa and
hop were prepared for electron microscopy and serially
sectioned from the tip. Tips of all terminal tracheids
narrowed rapidly towards the ends and no perforation
plates were observed. Tracheids ended either singly
in chrysanthemum, tomato and hop or in pairs in alfalfa.
Chrysanthemum and alfalfa endings were closely ad-
pressed to parenchyma cells with little discernable
distinction between the walls of both cells. This
intimate association suggested a free transfer of
xylem fluids through walls in the symplast. Tomato
and hop endings were positioned in areas of less
coherence being surrounded by many intercellular
spaces. All the tracheid tipswere triangular in cross-
section and extensively thickened. The endings con-
tained many pit surfaces and in chrysanthemum there
was a characteristic tail which appeared to be composed
of secondary thickened laminar fibres. Extensive
accumulation of granular phenolics in the lumens of
some tracheid tips suggested that this area was
particularly susceptible to aggregations of plant and
fungal metabolites which might contribute towards
water flow restriction to leaf mesophyll cells and
ensuing vascular wilt symptoms.
TUCKER, SHIRLEY C. Department of Botany,
Louisiana State University, Baton Rouge, LA
70803
- Character weighting in Leguminosae based on
time of initiation.
Unifying ordinal or familial characteristics are by
definition stable. These features should therefore
be determined early in floral ontogeny, while those
characteristics which separate related species
should occur relatively late in ontogeny. Examples
of early-determined features (to be illustrated with
examples from Leguminosae) include floral symmetry,
order of appendage initiation, number and kinds of
whorls, number of parts per whorl, and order of
initiation within a whorl. A second assemblage of
characteristics is determined in mid-development
stages; these include corolla aestivation, organ
abortion to produce "loss", elongation of some parts,
tube formation of calyx, corolla, and androecium,
petal fusion, differential growth of petals, differ-
ential growth of the two stamen whorls, and forma-
tion of gynophore and staminodes. Some of these
mid-development-determined characteristics are im-
portant in producing supra-generic distinctions,
while others are more important at the generic level.
Late-determined characteristics, which are likely to
distinguish related species or sometimes genera,
include differential changes in petal shape, petal
color, filament elongation, nectaries, nectar and
fragrance, epidermal elaborations (hairs, sculp-
turing, hooks, pits, cuticle), and changes in carpel
shape. Although exceptions abound to these gen-
eralizations, the hypothesis offers a useful ap-
proach to re-examining and evaluating diagnostic
characteristics in the light of their ontogenetic
origin.
Developmental and Structural Section 33
TUCKER, SHIRLEY C. Department of Botany,
Louisiana State University, Baton Rouge, LA
70803
- Developmental origins of petal aestivation in
Cadia purpurea and other legumes.
Although petal aestivation is used to separate sub-
families and some tribes of legumes, there is little
developmental evidence on its control in these
groups. Valvate aestivation typifies Mimosoideae,
ascending cochlear aestivation typifies Caesal-
pinioideae, and descending cochlear typifies Papi-
lionoideae. Petals are widely spaced and not imbri-
cate at initiation, nor do the bases extend margi-
nally to become imbricate. Overlapping occurs at
the margins about halfway up the petals when they
are about 300 pm high. Features which determine
which petal overlaps outside another include rela-
tive size, thickness, and degree of curvature of the
petal in transsection. Petals which overlap have
relatively attenuate margins, while those which ap-
press and fuse have thick, blunt margins. Cadia
purpurea, in the primitive papilionoid tribe Sopho-
reae, appears intermediate to the Caesalpinioideae
because of its highly variable petal aestivation.
Ascending cochlear and descending cochlear patterns
are both common among flowers on the same plant.
Also, two patterns occur which are rare in the le-
gumes: 1) a pattern in which the standard petal is
half inside, half outside the wings, and 2) com-
pletely quincuncial. Developmentally, Cadia's petal
enlargement is delayed greatly, compared to other
taxa. All the petals remain the same size and shape
throughout development. When the petals finally
approach one another, the pattern of overlap appears
to be a matter of chance, unlike the pattern in most
legumes.
VERBEKE, JUDITH and DAN B. WALKER. Department
of Biology, UCLA, Los Angeles, CA 90024
- Characterization of the mechanism underlying
induced epidermal dedifferentiation in the fusing
carpels of Catharanthus roseus.
In the process of floral ontogeny in Catharanthus
roseus approximately 400 epidermal cells are induced
to dedifferentiate during the postgenital fusion of
the two carpel primordia. Previously reported ex-
periments involving the placement of gold foil
barriers between prefusion carpels have showm that
dedifferentiation occurred only at points of direct
cell-to-cell contact. The morphogenetic stimulus in
this system may, therefore, consist of either a
diffusible messenger molecule or some kind of cell
surface interaction. To characterize the mechanics
of the induction of dedifferentiation we placed
barriers of various types between the pre-fusion
adaxial surfaces and monitored epidermal develop-
ment as the carpel faces grew into contact. Plastic
barriers (which permitted the diffusion Of 02 and
CO2 but which prohibited the diffusion of water)
blocked dedifferentiation of the contacting epider-
mal cells, giving a response similar to that pre-
viously reported with the gold foil barriers.
Polycarbonate membranes of knowm porosity which can
allow for passage of water soluble agents did not
block dedifferentiation of the contacting epidermal
cells. Thus, it appears that a diffusible agent is
involved in an intercellular communication that
triggers the dedifferentiation response. Current
efforts are aimed at better characterizing the
agent(s) .