12 Papaya
and branches, long petioles; widely evident, 40–60 cm diameter (Ming et al. 2008),
smooth, moderately palm shape with thick middle irradiant veins, the base is deeply
string shape with over imposed lobes; from 7 to 11 large lobed, each with a wide base
or slightly constrained and sharp-pointed, and sharp apex. The bundle of leaves is
dark green to yellowish green, bright, visibly marked by the off-white nerves embed-
ded and reticulated veins; the underneath surface is pale green-yellow and opaque
with visibly prominent vascular structures. Leaf blades are dorsiventral and sub-
tended by 30–105 cm long, hollow petioles that grow nearly horizontal (25–100 cm
length and 0.5–1.5 cm thick), endowed with a starch-rich endodermis, perhaps
important for cavitation repair (Leal-Costa et al. 2010). The leaf epidermis and the
palisade parenchyma are composed of a single cell layer, while the spongy meso-
phyll consists of four to six layers of tissue. Reflective grains and druses are abundant
throughout the leaf (Fisher 1980). Papaya leaves are hypostomatic with anomocytic
(no subsidiary cells) or anisocytic (asymmetric guard cells) stomata (Leal-Costa
et al. 2010). Stomatal density of sunlit leaves is approximately 400/mm^2 , which can
adjust readily to environmental conditions of light, water and heat. Important biolog-
ically active compounds have been identified in papaya leaves, where they function
in metabolism, defence, signalling and protection from excess light, among others
(Konno et al. 2004; Ming and Moore 2014).
2.1.3 Root
Papaya is very susceptible to wind break due to its flat root system, especially in the
monoculture. The papaya root is predominately a non-axial, fibrous system, com-
posed of one or two 0.5–1.0 m long tap roots. Secondary roots emerge from the
upper sections and branch profusely. These second-order feeding roots remain shal-
low during the entire life of the plant and show considerable gravitropic plasticity.
Many adventitious, lower order categories of thick and fine roots are also observed
in excavated specimens. Healthy roots are of whitish cream colour, and no laticifers
have been observed in them (Carneiro and Cruz 2009). The root phenotypic plastic-
ity is also high. The root size, number, distribution and orientation adjust readily
across the soil profile, to various soil conditions and throughout the life of the plant,
making papayas preferred components of complex agro-ecological models and hill-
side vegetation (Marler and Discekici 1997).
Leaves, stems and roots of young papaya plants exhibited rapid adaptive responses
to wind. Wind stress of only one week was sufficient to elicit significant responses.
Stem responses were the most rapid, but the root tip density responses exhibited the
greatest magnitude among all wind conditions. The asymmetry index is translated to
the windward side exhibiting 1.8–1.9 times more root tips than the leeward side. The
form of a root system becomes increasingly variable with the age of tree as it responds
to a number of stimuli and the bulk of the plant body increases (Marler 2011).
2.1.4 Flower
Plants are dioecious or hermaphroditic, with cultivars producing only female or
bisexual (hermaphroditic) flowers. Papayas are sometimes said to be ‘trioecious’