Zea mays

fully expanded leaves. Also a group of ccllulosesynthesising
enzymes located ar the plasmalemma.
Scale leaf A non-foliage leaf often investing dormant
buds or found in underground stems.
Schizogeny The separarion of cells along their middle
lamellae to form an intercellular sp:lce.
Sclereid A type of sclerenchyma cell characterised by its
very thick lignified walls and numerous pits; the shape
is variable but it is generally mueh shaner than a fibre.
Scferenchyma A supporting tissue whose cells arc
commonly dead at maturity and possess thick, lignified
secondary walls, as in fibres and sclereids.
Scutellum The highly modified cotyledon present in
grasses which supplies nutrienrs from the endosperm to
the germinating embryo.
SecondiJry cell wall The wall formed by the protoplast
at the end of expansion growth; the cellulose microfibrils
are closely crowded and, in anyone layer, lie
pacallel to each other (d., primary wall). Secondary
walls often become lignified, as in sderenchyma and
tracheary elements.
Secorulary thickemng This occurs in gymnosperms and
moSt dicotyledons and some anomalous monocotyledons.
The stem and root increase in diameter due to the
formation of sccond:lry vascular tissues by the vascular
cambium (or by the secondary thickening meristem),
whilst the epidermis is normally replaced by cork
formed from the cork cambium.
Secondary rhickening meristem In some arborescent
monocotyledons (e.g. Dracoeno, Cordyfine) an anomalous
form of secondary thickening occurs from a meristem
which arises in the outer cortex and Cuts off discrete
vascular bundles, plus parenchyma, centripeta11y.
Seed The structure which develops from the fertilised
ovule; it contains the embryo and a food supply to
support early seedling growth.
Sepal An outer perianth member which is distinct in
form from a petal.
Septate (ibre A fihre with thin cross walls which
develop after the longitudinal walls have become
thickened.
Sexine The outermost region of the pollen grains eetine
(d., nexine).
Shoot The non-rOOt region of the plant; it is usually
aerial and composed of the stem bearing numerous
photosynthetic foliage leaves (d., root).
Sielle area Modified pit fields in the side and oblique
end walls of sieve cells or rubes; the plasmodesmata
have been transformed into narrow sieve pores and
lateral translocation probably occurs via them.
Sieve cell The enucleate translocating element in
gymnosperms and lower vascular plants, possessing
sieve areas on all walls.
Sielle plates The transverse or somewhat oblique walls
occurring in sieve tubes; it conrains either a single series
of large pores or is compound with several series of
pores (ct., sieve area).
Sil've pore The hole in a sieve are;! or plate through
which cytoplasmic continuity occurs from one sieve
clement to another; the wall surrounding the pore is
commonly impregnated with callose.
Sieve tube An elongate clement comprising several to
many enucleate cdls interconnected via the sieve plates
(former cross walls). Sieve tubes are confined to
angiospenns (cL sieve cell).
Simple (ruit Formed from the single ovary of an
individual flower.
Softwood The wood of a conifer which generally lacks
thick-walled, lignified fibres; it is therefore easier to saw
than most hardwoods.
SpoJigy mesophyfl Very Irregular green parenchyma
cells with large intercellular spaces bC'twttn them; in
bilacial lea\'es this tissue occurs abaxiaUy (d., palisade
mesophyll).
Sporangium A .structure in which spores are produced;
in most fems the spores are uniform in size but in seed
plantS different sized spores are produced in mega- and
. .
mlerosporangl3.
Sporl's Haploid cells formed as derivatives of lhe
meiotic division of a diploid spore mother cell within a
sporangium. Each spore germinates to fOlm the
gametophyte.
Sporophyte The diploid phase of the life cycle; in
vascular plants the sporophpe is dominant (d.,
gametophyte).
Sporopo/lenin The substance composing the exine of
pollen grains; it is formed from cyclic alcohols and is
highly resistant to microbial decay.
Starch The chief food storage polysaccharide of plants
composed of several hundred hexose sugars; it is
insoluble and accuillulates within rhe stroma of various
plastids.
Starch sheath In many primary dicotyledonous stems
the inner layer of the conical parenchyma forms a
sheath with rich deposits of Starch in itS cells.
Stamelf The male organ of the flower composed of the
terminal amher bearing pollen and the basal sterile
filamenr.
Stigma The receptive zone of a carpel at the tip of the
style, upon which po11en is deposited and germinates.
Stipllfes Projections of tissue on either side of the base
of the leaf, which in dicotyledons :lre sometimes large
and vasculated.
Stoma (pI. stomata) A complex consisting of a pore in
the shOal epidermis which is surrounded by two
specialised guard cells; their turgidity causes the opening
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