<P> Additionally, structural proteins (1 - 5%) are found in most plant cell walls; they are classified as hydroxyproline - rich glycoproteins (HRGP), arabinogalactan proteins (AGP), glycine - rich proteins (GRPs), and proline - rich proteins (PRPs). Each class of glycoprotein is defined by a characteristic, highly repetitive protein sequence . Most are glycosylated, contain hydroxyproline (Hyp) and become cross-linked in the cell wall . These proteins are often concentrated in specialized cells and in cell corners . Cell walls of the epidermis may contain cutin . The Casparian strip in the endodermis roots and cork cells of plant bark contain suberin . Both cutin and suberin are polyesters that function as permeability barriers to the movement of water . The relative composition of carbohydrates, secondary compounds and proteins varies between plants and between the cell type and age . Plant cells walls also contain numerous enzymes, such as hydrolases, esterases, peroxidases, and transglycosylases, that cut, trim and cross-link wall polymers . </P> <P> Secondary walls - especially in grasses - may also contain microscopic silica crystals, which may strengthen the wall and protect it from herbivores . </P> <P> Cell walls in some plant tissues also function as storage deposits for carbohydrates that can be broken down and resorbed to supply the metabolic and growth needs of the plant . For example, endosperm cell walls in the seeds of cereal grasses, nasturtium and other species, are rich in glucans and other polysaccharides that are readily digested by enzymes during seed germination to form simple sugars that nourish the growing embryo . </P> <P> The middle lamella is laid down first, formed from the cell plate during cytokinesis, and the primary cell wall is then deposited inside the middle lamella . The actual structure of the cell wall is not clearly defined and several models exist - the covalently linked cross model, the tether model, the diffuse layer model and the stratified layer model . However, the primary cell wall, can be defined as composed of cellulose microfibrils aligned at all angles . Cellulose microfibrils are produced at the plasma membrane by the cellulose synthase complex, which is proposed to be made of a hexameric rosette that contains three cellulose synthase catalytic subunits for each of the six units . Microfibrils are held together by hydrogen bonds to provide a high tensile strength . The cells are held together and share the gelatinous membrane called the middle lamella, which contains magnesium and calcium pectates (salts of pectic acid). Cells interact though plasmodesmata, which are inter-connecting channels of cytoplasm that connect to the protoplasts of adjacent cells across the cell wall . </P>

Which of the following consists of an outer layer of dead cells and inner layers of new cells