Charest-Clark, Nancy. 1984. Preliminary scanning electron microscopic study of the peduncle, phyllary, and pale trichomes of Encelia (Asteraceae: Heliantheae). Crossosoma 10(4):1-6.
Preliminary Scanning Electron Microscopic Study of the Peduncle, Phyllary, and Pale Trichomes of Encelia (Asteraceae: Heliantheae)

by
Nancy Charest Clark

Biological Sciences
California State Polytechnic University,
Pomona CA 91768

Trichomes, or plant hairs, are among the most interesting features of plant surfaces. The woolly, velvety, or bristly appearance of many plants is a result of trichomes. Trichomes serve to protect the plant, either from the drying effects of wind and sun, or from plant-eating insects or other animals, which may be physically repelled by the hairs or poisoned by chemicals they produce.

The common desert shrub Encelia farinosa (brittlebush) has leaves with a silvery appearance. The thick woolly layer of trichomes that causes this appearance protects the leaves from the dry winds and bright sun of the desert, so that this plant is better able to withstand arid conditions. The price it pays is a reduction of photosynthesis, since the hairs reduce the amount of light reaching the leaves. For its coastal relative, E. californica, there is no need for this woolly covering, since it lives in moister environments.

Since leaf trichomes often differ among related species, they have often been used as a taxonomic character (Davis & Heywood, 1973). The leaf trichomes of Encelia were examined by Clark, et al. (1980). They described three types of trichomes, all multicellular, from the leaves: uniseriate hairs, moniliform hairs, and biseriate glandular hairs. Although they found similar trichomes in related species, the leaf trichomes were not as useful for taxonomic purposes as they had hoped.

They decided that this might be a result of the leaf trichomes of each species being specifically modified as an adaptation to the environment in which it grows. Since other parts of the plant are not so crucial for photosynthesis, it might be expected that their trichomes would be less modified (more "conservative"), and thus by retaining features in common with other species, be more useful for showing relationship.

The intent of this study is to examine the trichomes of three of these other parts, all associated with the composite flower heads: the peduncle (the modified stem supporting the head), the phyllaries (the reduced leaves surrounding the head), and the pales (the reduced leaves enfolding each flower in the head) (Fig. 1).

MATERIALS AND METHODS

Plant materials were obtained from 13 collections representing 12 taxa (Table I). Flower heads with peduncles were removed from plants in the field or in cultivation and placed in fixative, either FAA (formalin solution:acetic acid:70% ethanol, 5:5:90 v/v) or in 10% formaldehyde in pH 7.4 phosphate buffer. After fixation, specimens were washed in water and dehydrated in an ethanol series ending with 100% ethanol. Following dehydration, specimens were dried in a Sorval critical-point drier using carbon dioxide, mounted on aluminum specimen stubs with silver conductive paint, and coated with gold or gold-palladium alloy in a Technics Hummer. Specimens were observed with an AMR 1000 scanning electron microscope and photographed on Kodak 4127 sheet film.

Phylogenetic analysis was carried out using the WAGNER option of the PHYSYS program located on the California State University Central Cyber Computer.

RESULTS

hair typesThe three basic trichome types found on leaves (moniliform, glandular, and uniseriate hairs) are also found on peduncles, phyllaries, and pales, although, as in leaves, not all are found on each species. Examples of each of these are shown in Fig. 2 (left).

Whatever the complement of leaf trichomes for a species, it is uniform over the surface of the leaf (Clark, et al., 1980). This is also true of peduncles. Phyllaries and pales, on the other hand, have more localized distribution of certain trichomes. The margins and apices of phyllaries are often covered with very long uniseriate hairs (this giving the macroscopic appearance referred to as "ciliate"). The inner (adaxial) surfaces have trichomes only at the tip. On the outer (abaxial) surfaces, the trichomes are generally shorter and sparser than the margins, and if glands are to be found, they are usually found here. Pales are lacking trichomes altogether on the adaxial surfaces. At the apex of the abaxial surface is a tuft of trichomes, and these may continue a ways down the midrib.

Uniseriate hairs may be slender or broad-based, and the slender ones may be straight or curly; this variation is also found in leaf trichomes. Glandular hairs, that on leaves show great variation among the species in the size of their biseriate stalks, are much more uniform on peduncles, phyllaries, and pales. The distribution of the different trichomes is presented in Tables II (leaves), III (peduncles), IV (phyllaries), and V (pales).

DISCUSSION AND CONCLUSIONS

Although not all of the species have yet been examined, it is possible to draw some general conclusions. First, it is apparent that the leaf trichome complement of a species is not a good predictor of the peduncle, phyllary, or pale trichomes, nor are they good predictors of each other. In that the trichomes of these different organs likely serve different functions, this is not unexpected. Second, only on leaves are there always uniseriate hairs. Third, glands are far more common on these other structures than they are on leaves, whereas moniliform hairs are more common on leaves.

Two contrasting aims of taxonomy are the discovery of differences between species, so that they may be distinguished, and similarities, so that phylogenetic relationships between species can be understood. This study has provided both differences and similarities.

There is little difference in leaf trichome complement among E. palmeri, E. conspersa, and E. halimifolia. In fact, some botanists consider the latter two to be conspecific, since they show few other differences. The trichome complement of the pales also shows no difference, but there are differences among the species in both peduncles and phyllaries.

Differences alone, however, do not adequately show the relationships among the three species. Phylogenetic analysis, a method used by many contemporary taxonomists to show evolutionary relationship through shared derived features, was performed on the data from this study plus additional data from other parts of the plant used by C. Clark (pers. comm.) for phylogenetic analysis of the genus. Without the data provided by this study, the three species could not be distinguished, hence could not be analyzed. With the data included, analysis provided three different equally probable hypotheses of relationship (in an ideal analysis, there would be but one). So even though the added data help to resolve the situation, questions still remain.

Evidence provided by this study was more useful in resolving the relationships among the three varieties of Encelia farinosa. The added characters show that vars. farinosa and phenicodonta are closest relatives, and that var. radians is the next closest relative to these two.

Although the study was begun with the intent to record primarily the presence or absence of specific trichome types, this has proven in some cases to be misleading, since a specific trichome may constitute 75% of all trichomes in one species, and be rare in another. In addition, the distribution of trichome types on pales and phyllaries can differ between species. Because of this, I intend to calculate ratios of trichome types for the different organs of each species, and better characterize trichome distribution.

Even this preliminary study has provided useful information. On completion, it will help us better understand the evolution of trichomes in the genus and in conjunction with other studies enable a more thorough analysis of phylogeny.

ACKNOWLEDGEMENTS

This project was funded by a grant from Southern California Botanists. I thank Curtis Clark for his assistance with many parts of this study, and I am especially grateful to my seven week old daughter Emily, without whose help this could have been written.

REFERENCES CITED

Clark, C., W.C. Thompson, and D.W. Kyhos. 1980. Comparative morphology of the leaf trichomes of Encelia (Compositae: Heliantheae). Botanical Society of America, Misc. Publ. 158.

Davis, P.H., and V.H. Heywood. 1973. Principles of angiosperm taxonomy. Krieger. 559 pp.



TABLE I
Sources of Specimens

Encelia asperifolia (Blake) Clark & Kyhos:
132--MÉXICO. BAJA CALIFORNIA: Millers Landing, behind dunes, Mar 1978.

Encelia californica Nutt.:
166--UNITED STATES. CALIFORNIA. Ventura Co.: CA Hwy 126, 4.6 mi E of Piru, 18 May 1980.

Encelia canescens Lam.:
1755--PERÚ. AREQUIPA: Near Atiquipa. Charles Rick coll., 19 Jan 1979.

Encelia conspersa Benth:
189--MÉXICO. BAJA CALIFORNIA SUR: San Carlos, N of town on old dunes, 26 Mar 1981.

Encelia farinosa Gray var. farinosa:
018--UNITED STATES. CALIFORNIA. San Bernardino Co.: Whitewater Wash, D.W. Kyhos coll., Mar 1965.

Encelia farinosa Gray f. phenicodonta Blake:
186--MÉXICO. BAJA CALIFORNIA SUR: S of Bahía Concepción at Microondas Rosarito, 25 Mar 1981.

Encelia farinosa Gray var. radians Brandegee:
203--MÉXICO. BAJA CALIFORNIA SUR: Road to La Ribera, 0.7 mi E of Mex Hwy 1. Mesquite grove. 22 Mar 1982.

Encelia frutescens (Gray) Gray var. resinosa Jones:
195--UNITED STATES. ARIZONA. Coconino Co.: Az Hwy 64, 13.3 mi W of jct with US Hwy 89 at Cameron, 5000 ft, 14 Jun 1981.
226--UNITED STATES. UTAH. Grand Co.: On UT Hwy 313, 11.8 mi N of road into Canyonlands; and 2.6 mi SW of US 191. Clark 601. 21 June 1983.

Encelia halimifolia Cav.:
143--MÉXICO. Sonora: Ann Johnson 4128.

Encelia palmeri Vasey & Rose:
118--MÉXICO. BAJA CALIFORNIA SUR: near Hwy 1 ca. 15 mi S of Guerrero Negro, 7 May 1978.

Encelia ravenii Wiggins:
164--MÉXICO. BAJA CALIFORNIA: 8.4 mi W of Mex Hwy 5 and San Felipe on road to Valle de San Felipe, Eloy Rodriguez coll., 19 April 1980.

Encelia virginensis A. Nels.:
216 vir--UNITED STATES. CALIFORNIA. San Bernadino Co.: On Black Canyon Road, 3.5 mi N of jct with Essex Road in Colton Hills area. 14 May 1983.



                            TABLE II
               Leaf trichomes of examined species
----------------------------------------------------------------------
Trichome type    132 166 1755 189 018 186 203 195 226 143 118 164 216
----------------------------------------------------------------------

straight
uniseriate        -   +   +    +   -   -   -   -   -   +   +   +   +

curly
uniseriate        -   -   +    -   +   +   +   -   -   -   +   -   -

broad-based
uniseriate        +   -   -    -   -   -   -   +   +   -   -   -   +

moniliform        +   +   +    +   -   -   -   -   +   +   +   -   -

glands            +   -   -    -   -   -   +   +   +   -   -   +   +

----------------------------------------------------------------------
C. Clark, 1984, pers. comm.



                            TABLE III
             Peduncle trichomes of examined species
----------------------------------------------------------------------
Trichome type    132 166 1755 189 018 186 203 195 226 143 118 164 216
----------------------------------------------------------------------

Straight
uniseriate        +   -   +    -   -   -   -   -   -   -   -   -   -

Curly
uniseriate        -   +   -    -   +   +   -   -   -   -   +   -   -

Broad-based
uniseriate        -   -   -    -   -   -   -   -   +   -   -   -   -

moniliform        +   +   +    -   -   -   -   -   -   +   -   -   -

glands            +   +   -    +   -   +   +   +   +   +   +   +   +

----------------------------------------------------------------------



                            TABLE IV
             Peduncle trichomes of examined species
----------------------------------------------------------------------
Trichome type    132 166 1755 189 018 186 203 195 226 143 118 164 216
----------------------------------------------------------------------

Straight
uniseriate        +   -   +    -   -   -   -   -   -   -   -   -   -

Curly
uniseriate        -   +   -    -   +   +   -   -   -   -   +   -   -

Broad-based
uniseriate        -   -   -    -   -   -   -   -   +   -   -   -   -

moniliform        +   +   +    -   -   -   -   -   -   +   -   -   -

glands            +   +   -    +   -   +   +   +   +   +   +   +   +

----------------------------------------------------------------------



                             TABLE V
               Pale trichomes of examined species
----------------------------------------------------------------------
Trichome type    132 166 1755 189 018 186 203 195 226 143 118 164 216
----------------------------------------------------------------------

Straight
uniseriate        +   +   -    +   -   -   +   +   -   +   +   -   -

Curly
uniseriate        -   -   -    -   +   +   -   -   -   -   -   -   -

Broad-based
uniseriate        -   -   -    -   -   -   -   -   +   -   -   -   -

moniliform        -   -   +    -   -   -   -   -   -   -   -   -   -

glands            +   +   +    +   +   +   +   +   +   +   +   +   +

----------------------------------------------------------------------

Copyright © 1984 by Nancy A. Charest. Reprinted with permission.