Primary Growth of Stems

                                                                                                                             

I.              Main functions of stems

                A.            Support

                B.            Conduction of water and nutrients

                C.            Storage

                D.            Production of new living tissue

II.            Primary stem growth in angiosperms  - dicots and monocots

                A.            FOCUS ON DICOTS

                                1.             What gives rise to different tissues?

                                                a)             Meristems

                                                b)             Cells differentiate - become destined to dermal, ground, or vascular

                                                c)             protoderm = dermal;  ground mer. = ground;  procambium = vascular

                                2.             Apical meristem - angiosperms - tunica & corpus structure

                                                a)             Shoot apex - point above youngest primordia (lateral outgrowth from apical meristem that will become a leaf

                                                b)             Promeristem - place where no visible differentiation can be seen (can be same as shoot apex)

                                                c)             Very top cells comprise the tunica; other cells corpus

                                                                1)            Tunica

                                                                                a)             Gives rise to the protoderm, i.e., epidermis

                                                                2)            Lower Tunica & Corpus

                                                                                a)             Gives rise to ground meristem and procambium, i.e., ground & vascular tissues

                                3.             Differentiation of cells

                                                a)             Dermal tissue

                                                                1)            Epidermis - covered by a cuticle

                                                                2)            Can have chloroplasts and starch grains in the chloroplast

                                                b)             Ground tissue

                                                                1)            All around procambial strands

                                                                2)            Two parts

                                                                                a)             Cortex - outer part between procambial strands & epidermis

                                                                                b)             Pith - inside region

                                                c)             Procambial strand

                                                                1)            Phloem - outside & continuous

                                                                                a)             Called protophloem - later becomes crushed

                                                                2)            Xylem - inside & discontinuous (no immediate need for water)

                                                                                a)             Called protoxylem - later becomes torn apart (monocots -protoxylem lacuna looks like a big hole) or obliterated (dicots)

                                4.             Arrangement of vascular bundles - DICOTS

                                                a)             Originate as ring of "residual mer." that differentiates - xylem & phloem

                                                b)             Bundles become separated by regions of parenchyma (pith rays)

                                                c)             Bundles are arranged in a ring-like structure

                B.            DIFFERENCES BETWEEN DICOTS AND MONOCOTS

                                1.             Arrangement of vascular bundles is scattered

                                2.             Pith and cortex cannot be defined

                                3.             Very few species undergo secondary growth

                                                a)             Most grasses lack a vascular cambium

III.           Additional primary growth of vascular tissue - DICOTS AND MONOCOTS

                A.            Phloem - conduction of organic stuff (sugars)

                                1.             Protophloem - permits conduction of organics during rapid growth - later crushed

                                2.             Metaphloem - matures and functions after rapid growth

                B.            Xylem - conduction of water and mineral nutrients

                                1.             Protoxylem - permits conduction of solubles during rapid growth - later gets torn apart to form a lacuna (monocots) or obliterated (dicots)

                                2.             Metaxylem - matures and functions after rapid growth