Fabaceae is regarded as the modern and accepted name of the family of
which Mimosa belongs (Marshman, 2004). The subfamilies are Mimosoideae,
Caesalpiniodeae and Papilionoideae (which is now regarded as Faboideae).
Vidyarthi and Tripathi (2002) reported that the subfamily has 56 genera
and 2000 species, which are commonly found in tropical and subtropical
The Mimosa species are characterized by their nyctinastic movement.
It is seen most especially and rapidly in M. pudica (Arbonnier,
2004). The nyctinastic movement (opening and closing) of the leaves is
a circadian phenomenon and persists even when plants are kept continuously
in light or darkness. M. pudica is a widespread weed of native
America but was introduced to West Africa in 1839 and 1840 (Burkill, 1995).
Mimosa invisa Mart. commonly called giant sensitive plant is a
biennial or perennial legume. It starts as an erect plant and grows into
a spiny climbing shrub with the capacity to smoother competing plants
in its domain. It has a variety known as M. invisa var Inermis
Adelb. They are similar but can be distinguished by the absence of
prickles on the stem, leaves and fruits of the variety Inermis. Burkill
(1995) described M. invisa as one of the worst weeds due to its
nasty thorns. According to Alabi et al. (2001), M. invisa limits
the yield of cassava (Manihot sp.) in South Western Nigeria where
more than 30% of total national production is generated.
M. pigra often found in riverbanks and fresh water swamps has
been found common throughout the regions from Senegal to Nigeria and widespread
over tropical Africa. It is also known as M. asperata Linn. (Burkill,
1995). M. pigra is an invading woody, prickly, leguminous shrub
and it is capable of a dramatic population explosion (Miller and Lonsdale,
1993). M. pigra is similar in appearance to M. invisa.
It can be distinguished by its very sensitive leaves, very strong
thorns and pale mauve flower balls which differ from the pinkish flower
of M. invisa.
This study is carried out with a view to using available differences
in the micromorphology of the leaf cuticle in characterizing these three
common and economically important Mimosa species available in Nigeria.
MATERIALS AND METHODS
This study was carried out between January and July, 2005 at Michael
Okpara University of Agriculture, Umudike, Nigeria. The fresh leaves were
collected from plants growing in natural conditions in different locations
of Eastern Nigeria. The leaf samples were boiled in concentrated HNO3
for 2 min. The samples were then carefully washed in water and the adaxial
and abaxial epidermis teased from the mesophyll using fine forceps and
dissecting needles. The peelings of the leaves were then mounted in glycerine
on glass slides.
Photomicrographs were taken using a Leitz Wetzler Ortholux microscope
fitted with a Vivitar-V335 camera.
The characteristic epidermal features of the three taxa are shown in
Fig. 1-2 and Table 1. The morphological
type of stomata found in the three Mimosa species is the
|(a) Epidermal morphology showing diacytic stomatal arrangement
in lower epidermis of M. pudica. (b) Upper epidermal cells
of M. pudica with irregularly shaped epidermal cell walls.
(c) Lower epidermis of M. pigra with irregularly shaped epidermal
||Epidermal characters of the Mimosa species studied
diacytic stomata type. The epidermal cells were mostly irregularly shaped,
but the epidermal cell walls of M. invisa were mostly rectangular
to pentagonal in shape (Fig. 2a, b).
|(a) Lower Epidermis of M. invisa with polygonal shaped
cells. (b) M. invisa showing less number of stomata on upper epidermis.
(c) Irregularly shaped epidermal cell walls of upper epidermis of
The anticlinal cell walls of the three taxa were straight in nature.
Stomata was present on both the lower and upper epidermis of all the species
investigated, i.e., they were amphistomatic. From data collected on the
stomata index, stomata were more on the lower epidermis than on the upper
epidermis. The stomatal index in the lower epidermis varied from 22.95%
(M. pigra through 13.43 (M. invisa) to 24.39% in M. pudica.
Leaves are probably the most varied organs of the angiosperms. Carlquist
(1961) has reported the valuable role of leaf anatomy in taxonomy. Irvine
(1961), Metcalfe and Chalk (1983) have reported the use of epidermal characters
such as leaf surface, epidermal cell wall pattern, nature of stomata as
identifiable aids of some families and genera and sometimes for species.
In the presently investigated Mimosa species, the epidermal cell
walls varied from irregular in shape to rectangular or pentagonal in both
the upper and lower epidermis.
The stomata type was diacytic in all species. Remarks have been made
by some researchers in different plants on the use of epidermal characters
in taxonomy (Edeoga and Ugbo, 1997; Edeoga and Ikem, 2002; Chandra et
al., 1969). Stomatal frequency is one of the most widely used characters
in taxonomy and pharmacognosy. (Karishnamurthy and Sundaram, 1970). Many
other researchers like Ahmad (1964) have established the significance
of stomatal frequency as a taxonomic tool. In the Mimosa species
studied, the stomatal frequency varied significantly. The very low occurrence
of stomata on the upper epidermis of M. invisa and the relatively
high stomatal index of the lower epidermis of M. invisa also can
be conveniently used as a distinguishing character from other Mimosa
The results obtained from the study of the micromorphology of the three
Mimosa species will help in separating these taxa from other similar
species or related taxa. This study, apart from the general morphology
of these plants will help in the recognition of the Mimosa species
of economic and medicinal importance. It will also contribute to the biology
of the obnoxious species of Mimosa.
From the shape of the epidermal cell wall, M. invisa could be
separated from the three taxa due to its rectangular-pentagonal shaped
epidermis. This is in line with earlier affirmation by other researchers
(Chandra et al., 1969). Similarly, M. pigra could be separated
from M. pudica due its relatively lower value of stomatal index
(22.95) as against the relatively high stomatal index of M. pudica
(27.50). This is not strange as well since similar observations have
been made by other researchers in other taxa (Edeoga and Jimoh, 2005;
Chandra et al., 1969).
An interesting area of further research is that of their exhibition of
nyctinastic movement by these taxa. Differences in leaf epidermal micromorphology
could be involved in the various degrees of response to the sense of touch
by these Mimosa species, investigated. However, further research
is required in this direction so as to understand clearly the probable
role of leaf characters in nyctinastism.