Flower development of Cannabaceae species using SEM
Authors: Leme Flávia Maria , Teixeira Simone Pádua 
Published: December 2016

Developmental studies are important to understand the morphology and evolution of the flower, an exclusive unique structure of the Angiosperms. For such studies, the analysis in SEM is essential to the understanding of organogenesis and organ initiation order. The Cannabaceae family, well known to include species such
Humulus lupulus
(hop) and
Cannabis sativa
(hemp), recently had its circunscription expanded, with the addition of some genera (e.g.
Celtis
and
Trema
) before belonging to Ulmaceae family. The flowers of these four genera have some diagnostic differences (dicliny, monocliny), but share some conditions (perianth single, few stamens, pseudomonomerous gynoecium) which causes the authors to consider them reduced compared to other related families. Then, the aim of this work was to study the development pathways that result in flower of the
Celtis iguanaea
(Jacq.) Sarg. and
Trema micrantha
(L.) Blume using SEM and compare them to the data available for
Cannabis sativa
and
Humulus lupulus
. From this comparison it will be possible to infer if the development of the flower is homologous in this group, which would support the inclusion of
Celtis
and
Trema
in Cannabaceae. To this end, flowers in various stages of development were fixed in buffered formalin, dehydrated in an ethanol series, critical‐point dried, mounted on aluminum stubs with carbon tape, and coated with gold of 85,5 nm thick in current of 40mA. Samples were analyzed and documented in SEM (Zeiss IVO‐50) with voltage 10 kV. Interpretations of electron micrographs shows that the species analyzed have similar floral development. The floral meristem is rounded and subtended by an abaxial bract and two small lateral bracts (figure 1). The floral meristem iniates five sepals, five opposite stamens and a carpel primordia. The order of sepal and stamen initiation is helical (figure 2) and none of the species presents petals from the inception (figure 2), resulting in flowers single perianthed. The pistil starts its development as a single primordium (figure 2), which subsequently divides into two, giving rise to two carpels (figure 3 and 4); however, only one is responsible for the formation of the ovule. This pathway gives rise to the pseudomonomerous gynoecium. In
Trema micrantha
, a dioecious species, the dicliny occurs by abortion of the pistil in the staminate flowers and of the stamens in the pistillate flowers (figure 3). In
Celtis iguanaea
, an andromonoecious species, occurs abortion of the pistil in the staminate flowers (figure 4). No pistillate flowers were found in studied plants (figure 4), but perfect (=bisexual) flowers. Our results show that
Celtis iguanaea, Trema micrantha, Cannabis
sativa and
Humulus lupulus
do not have similar floral development. Only the absence of petals is shared among the four species of the four genera. The dicliny occurs by abortion in
Celtis
and
Trema,
and by inception in
Cannabis
and
Humulus
. The beginning of pistil development is also different: a single primordium initiates in
Celtis
and
Trema
while two primordia initiate in
Cannabis
and
Humulus
. Thus, we can conclude that the floral developmental features do not support the phylogenetic positioning of
Celtis iguanaea
and
Trema micrantha
in Cannabaceae. Different developmental vias give rise to the reduced flower in this group.