SUMMARY Secretory tissues occur in most vascular plants. Some of these tissues, such as hydathodes, salt glands and nectaries, secrete unmodified or only slightly modified substances supplied directly or indirectly by the vascular tissues. Other tissues secreting, for instance, polysaccharides, proteins and lipophilic material, produce these substances in their cells. The cells of secretory tissues usually contain numerous mitochondria. The frequency of other cell organelles varies according to the material secreted. In most glandular trichomes the side wall of the lowest stalk cell is completely cutinized. This prevents the secreted material from flowing back into the plant. The salt glands in Atriplex eliminate salt into the central vacuole of the bladder cell but, in other plants, the glands secrete salt to the outside. Different views exist as to the manner in which salt is eliminated from the cytoplasm. According to some authors, the mode of elimination is an eccrine one, while others suggest the involvement of membrane‐bound vesicles. Nectar is of phloem origin. The pre‐nectar moves to the secretory cells through numerous plasmodesmata present in the nectariferous tissue. Nectar is eliminated from the secretory cells by vesicles of either KR or dictyosomal origin. In some cases, both organelles may be involved but an eccrine mode of nectar secretion has also been suggested by some authors. Carbohydrate mucilages and gums are synthesized by dictyosomes but virtually every cell compartment has been suggested as having a role on the secretion of lipophilic substances. Most commonly, plastids are implicated in the synthesis of lipophilic materials but KR may also play a part. In some cases lipophilic materials may be transported towards the plasmalemma in the KR. Resin and gum ducts of some plants develop normally or in response to external stimuli, such as microorganisms or growth substances. Among the latter, ethylene is the most effective. During the course of evolution, secretory tissues seem to have developed from secretory idioblasts scattered among the cells of the ordinary tissues. Subsequently ducts and cavities developed and finally secretory trichomes.

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Data on vessel arrangement, grouping, diameter, perforations, element length, wall thickness and helical thickenings are presented for the woody flora of Israel and adjacent regions. Within this flora the arid, Mediterranean, hygrophylic and synanthropic components are compared with each other and with information on woods from the tropical rain and monsoon forest flora of Java and the mesic cool temperate flora of North West Europe.In these floristic comparisons, vessel element length decreases from mesic to xeric vegetations and within mesic floras from tropical to cool temperate or arctic latitudes. Other characters show complex trends and individual ecological categories show very wide, overlapping ranges of wood structural possibilities. The arid flora shows the highest proportion of species with a combination of numerous narrow, and wide vessels in their wood; a high degree of vessel grouping is also most common in this ecological category. Tropical species surviving in wadis of the Negev and Sinai desert do not show this syndrome but resemble their relatives from less xeric conditions. The wood of arid species in general appears to be at least as well adapted to efficient water transport (high values for maximum vessel diameter) as to safety (provided for by numerous, narrow vessels). Vessel walls tend to be thickest in the arid flora. Spiral thickenings are most common in the Mediterranean and temperate floras and are quite infrequent in both the arid subtropical flora of the Middle East and the mesic to seasonally dry tropical flora.Factors underlying the trends observed can only partly be retraced to ecological tendencies within genera; partly they appear to depend on the predominance of species belonging to specialised families in extreme habitats like the desert.

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