Information on the sites of cellulose synthesis and the diversity and evolution of cellulose-synthesizing enzyme complexes (terminal complexes) in algae is reviewed. There is now ample evidence that cellulose synthesis occurs at the plasma membranebound cellulose synthase, with the exception of some algae that produce cellulosic scales in the Golgi apparatus. Freeze-fracture studies of the supramolecular organization of the plasma membrane support the view that the rosettes (a six-subunit complex) in higher plants and both the rosettes and the linear terminal complexes (TCs) in algae are the structures that synthesize cellulose and secrete cellulose microfibrils. In the Zygnemataceae, each single rosette forms a 5-nm or 3-nm single ''elementary'' microfibril (primary wall), whereas rosettes arranged in rows of hexagonal arrays synthesize crisscrossed bands of parallel cellulose microfibrils (secondary wall). In Spirogyra, it is proposed that each of the six subunits of a rosette might synthesize six ␤-1,4-glucan chains that cocrystallize into a 36glucan chain ''elementary'' microfibril, as is the case in higher plants. One typical feature of the linear terminal complexes in red algae is the periodic arrangement of the particle rows transverse to the longitudinal axis of the TCs. In bangiophyte red algae and in Vaucheria hamata, cellulose microfibrils are thin, ribbon-shaped structures, 1-1.5 nm thick and 5-70 nm wide; details of their synthesis are reviewed. Terminal complexes appear to be made in the endoplasmic reticulum and are transferred to Golgi cisternae, where the cellulose synthases are activated and may be transported to the plasma membrane. In algae with linear TCs, deposition follows a precise pattern directed by the movement and the orientation of the TCs (membrane flow). A principal underlying theme is that the architecture of cellulose microfibrils (size, shape, crystallinity, and intramicrofibrillar associations) is directly related to the geometry of TCs. The effects of inhibitors on the structure of cellulose-synthetizing complexes and the relationship between the deposition of the cellulose microfibrils with cortical microtubules and with the membrane-embedded TCs is reviewed In Porphyra yezoensis, the frequency and distribution of TCs reflect polar tip growth in the apical shoot cell. 1The evolution of TCs in algae is reviewed. The evidence gathered to date illustrates the utility of terminal complex organization in addressing plant phylogenetic relationships.

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Glandular scales of Origanum dictamnus L. originate from a single protodermal cell. They are composed of a 12-celled head and an unicellular stalk and foot. During the early stages of gland differentiation, the head cells possess a small number of plastids which contain globular inclusions. Similar inclusions are also observed in the plastids of the stalk and the foot cell. The lateral walls of the stalk cell progressively undergo cutinization which does not extend to the upper and lower periclinal walls. At the onset of secretion the electron density of the plasmalemma region lining the apical walls of the head cells remarkably increases. These walls are impregnated with an osmiophilic substance identical in appearance to the content of the subcuticular space. In a following stage of the secretory process osmiophilic droplets of various size arise in the cytoplasm of the secretory cells which undergoes simultaneously a reduction of its initial density. After secretion has been concluded the protoplast of the head cells becomes gradually degenerated. The chlorenchyma cells of the mesophyll possess numerous microbodies closely associated with various organelles. In the cytoplasm of these cells crystalloids occasionally occur.

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Tsekos, I. 1989. The floral nectaries of Hibiscus rosa-sinensis 111. A morphometric and ultrastructural approach. -Nord. J. Bot. 9: 63-71. Copenhagen. ISSN 0107-055X.The secretory hairs of Hibiscus rosa-sinensis nectaries have been studied ultrastructurally, particularly at stages before, during and after secretion. Morphometric cytology revealed considerable changes in the volume of endoplasmic reticulum (ER) and the vacuoles. At least three different forms of ER have been noticed, cisternal, tubular and vesicular. The intermediate cells of the hairs are ultrastructurally and morphometrically similar to the tip cell, suggesting their probable involvement both in a symplastic prenectar transport via the plasmodesmata and in nectar release into an extracellular space provided by the lateral cell walls. Nectar would then be apoplastically moving towards the tip cell, where it is forcibly expelled to the outside via transient pores of the cuticle. Ultrastructural evidence indicates that ER is the cell compartment principally involved both in prenectar transport and nectar elimination. In the hair cells it provides a "secretory reticulum'' mediating between prenectar accumulation and nectar release. Vacuoles are voluminous in the basal cells, and in all cell types of old nectaries. The results are discussed in relation with other plant glands, especially with the closely related Abutilon nectaries.