Abstract
The purpose of this paper was to demonstrate the diversity in nonglandular trichome morphology of tarweeds at the ultrastructural level, and to propose categories for the morphologies reported for such trichomes. Using light and scanning electron microscopy (SEM), the authors surveyed 31 species of II genera of tarweeds from California and three genera of tarweeds from Hawaii. Trichome cell wall thickness in the Californian species ranged from 1 to 6 ~μm, whereas in the Hawaiian species trichome cell wall thickness ranged from 1 to 14 ~μm. Based on their surface appearance using SEM, trichomes were grouped into four categories: grooved; smooth + grooved; smooth; and verrucose. The verrucose surface of trichomes is an innate feature of the cell wall and is not due to resinlike deposits. The ecology and growth form of the plants in each group were compared to the trichome surface structure. Ecologically, members of the grooved group were alike in experiencing prolonged exposure to solar irradiation; eight of the nine plants in this group are perennial. The smooth + grooved group of trichomes was found on tarweeds that inhabit various types of habitats. This is likely a transitional group. The smooth trichome group is composed of mostly low-elevation Californian annuals. No Hawaiian tarweed surveyed had completely smooth trichomes. Tarweeds having verrucose-surfaced trichomes were mostly found in regions of moderate moisture. The trichomes observed in Dubautia platyphylla were unique and could not be placed into any of the four groups; the surface of these trichomes was a combination of verrucose and smooth.
Highlights
Recent chloroplast DNA data indicate that the mainland tarweeds of southwestern United States gave rise to the Hawaiian tarweeds (Asteraceae tribe Heliantheae, subtribe Madiinae) (Baldwin, Kyhos, Dvorak, and Carr 1991)
The role ofthe founder effect is expressed in the limited gametic chromosomal number of the Hawaiian tarweeds, as compared to the immense range ofmainland tarweeds
In our survey of Hawaiian and Californian tarweed trichomes, we examined 31 species using the scanning electron microscope (SEM)
Summary
Recent chloroplast DNA data indicate that the mainland tarweeds of southwestern United States gave rise to the Hawaiian tarweeds (Asteraceae tribe Heliantheae, subtribe Madiinae) (Baldwin, Kyhos, Dvorak, and Carr 1991). With reference to the evolutionary diversification of Hawaiian tarweeds, there may have been important founder effects associated with each of the estimated 14 major dispersal and colonization events (Robichaux, Carr, Liebman, and Pearcy 1990). The ecology of the mainland and Hawaiian tarweeds is quite different. The Hawaiian tarweeds inhabit lava flows, cinder cones, dry scrub, dry forests, mesic forests, rainforests, and bogs (Kyhos et al 1990). The diverse leaf length (5-500 mm; Robichaux et al 1990) of the Hawaiian tarweeds can be related to the diverse ecology of these plants. The morphology of the trichomes of the Hawaiian tarweeds may result from founder effect. The role ofthe founder effect is expressed in the limited gametic chromosomal number of the Hawaiian tarweeds, as compared to the immense range ofmainland tarweeds.
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