TRICHOME MORPHOLOGY IN HEDERA ( ARALIACEAE )

Trichomes of Hedera L. taxa have long been used as an important character in delimiting species. Hedera exhibits two distinct trichome types: scale-like and stellate. This study examined the trichome variation in sixteen currently recognized Hedera taxa using scanning electron microscopy. Measurements of trichome morphology were taken, and variation among taxa compared. Of the taxa with scale-like trichomes, H. maderensis Rutherford subsp. maderensis has the largest overall length, longest rays, and largest fusion of rays. Of the species with stellate trichomes, H. helix has the largest overall, and also has the longest rays and widest rays.


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The material used in the study was obtained from both herbarium specimens and living material. The living material was obtained through the American Ivy Society, Lewis Ginter Botanic Garden, and Dr Hugh McAllister at Ness Gardens. The leaves most suitable for use in the study were located two or three leaves down on the plant. Both the abaxial and adaxial leaf surfaces of juvenile plants were studied. No discernable differences in measurements were found between the abaxial and adaxial trichomes.
Samples of approximately 7×7mm were taken from herbarium specimens and leaf samples (see Table 1 for sample origin). These were attached to an aluminium stub with double-sided tape, and coated with gold using a Hummer VII Sputtercoater. Living material was attached Richmond, VA #220 to stubs with silver paste and allowed to air dry for 24h before being coated with gold. The samples were observed using a Philips 505 Scanning Electron Microscope. The measurements taken from the trichomes (see Fig. 1) were as follows: A, radius of raised centre; B, length of ray from edge of raised centre; C, length of fusion of rays from edge of raised centre; D, length of base; and E, ray width. Calculation 2 (A+B) was used as the total length of the trichome. Three measurements of each length were taken on the same trichome. At least 10 trichomes on each leaf sample were measured. Two samples of each trichome were examined (Table 1), except in the case of H. cypria where material was limited. MINITAB (1997) was used for all statistical analyses. Table 2 lists the measurements taken for the Hedera species examined. For the scalelike trichomes (Fig. 2), there are various key characters that can be used to separate trichomes of several taxa. Hedera cypria can be easily distinguished from other taxa as its trichomes only appear on the abaxial leaf surfaces. This is the only taxon in which the trichomes are not distributed on both the adaxial and abaxial leaf surfaces. The trichomes of H. colchica are also easily distinguished from other scale-like trichomes as they have the largest centre length (0.036-0.048mm), which is readily seen under a light microscope. These trichomes also usually have the highest number of rays (up to 20) present in any scale-like trichomes. Hedera maderensis subsp. maderensis has the largest average overall size (0.225-0.377mm), longest rays (0.088-0.164mm), and greatest fusion of rays (0.025-0.052mm) of the scale-like trichomes. The trichomes of H. maderensis subsp. iberica have a larger raised centre (up to 0.094mm across) than those of H. maderensis subsp. maderensis.

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The  comparison. These trichomes most closely resembled the scale trichomes, but lacked a definite centre (Fig. 3). However, not all species with scale-like trichomes can be distinguished on trichome morphology. Hedera canariensis and H. algeriensis have very similar trichomes and cannot be separated on trichome characters alone. The trichomes of both these species have small centres (<0.05mm in diameter) and an average overall length of approximately 0.25mm. Likewise, the trichomes of H. nepalensis, H. nepalensis var. sinensis, and H. pastuchovii are all very similar and difficult to separate. The trichomes of all these taxa all have large centres (>0.05mm in diameter) and are relatively small in overall length (<0.25mm).
In contrast, taxa with stellate trichomes (Fig. 3) can be separated solely on trichome morphology. Hedera helix forma poetarum can be recognized from closely related taxa by the long base (generally >0.05mm) attaching the trichome to the leaf surface. The trichomes of H. helix subsp. rhizomatifera can be easily distinguished from other stellate trichomes because they almost always have only four rays arranged opposite each other, giving the trichome a cruciform appearance. The trichomes also lie more adpressed to the leaf surface than those of the typical subspecies. Trichomes of H. azorica have a short base (<0.02mm long).
Distinguishing between H. helix and H. hibernica is generally difficult, but one of the few characters that can be used to separate them is the position of the trichomes on the leaf surface. Results confirm the findings of McAllister (1990) that the trichomes of H. hibernica generally lie parallel to the leaf surface while those of H. helix are erect and more bristly in appearance. Furthermore, the trichomes of H. helix are generally much larger overall (0.75-1.062mm) than those of H. hibernica (0.4-0.875mm), and the base of H. helix trichomes are longer (0.035-0.05mm) compared with H. hibernica trichomes (0.013-0.025mm).
These results have been used to create the following key: This study has shown the usefulness of trichome morphology in Hedera for delimiting taxa. Although there are taxa which cannot be determined solely using on trichome morphology, trichome characters can be a very useful tool in distinguishing between many of the currently recognized Hedera taxa.

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I would like to thank Dr Jun Wen for advice, suggestions, and comments, and Dr John Chandler for his assistance in using the electron microscope. I would also like to thank the American Ivy Society and Lewis Ginter Botanic Garden for supplying leaf material, Dr Hugh McAllister for his assistance at Ness Gardens, and the National Science Foundation (DEB9726830, to Dr Jun Wen), the Linnaean Society