Why are there so few hard leaves in the daisy family?
(writing in progress)
The daisy family (Asteraceae, https://en.wikipedia.org/wiki/Asteraceae and https://keys.lucidcentral.org/keys/v3/FFPA/key/FFPA/Media/Html/Asteraceae.htm#:~:text=Pollination%20by%20insects%20or%20rarely,absent%20or%20on%20the%20carpels. and https://www.gbif.org/species/113579929) presents something of a paradox in terms of leaf-texture.
This family contains more species than any other dicotyledonous family, worldwide. And its adaptive radiation has produced ephemerals, annual herbs, geophytes (https://annals.mobot.org/index.php/annals/article/view/340), succulents (https://en.wikipedia.org/wiki/Kleinia), lianes (https://www.researchgate.net/publication/226787219_Vernonia_excelsa_Compositae_a_new_liana_from_Central_Africa_and_notes_on_related_species), shrubs, and trees (https://en.wikipedia.org/wiki/Brachylaena and https://www.sciencedirect.com/science/article/pii/S1870345313728653#:~:text=The%20tallest%20tree%20species%20reach,67%20tree%20species%20in%20Mexico.), with a range of diaspores, and mechanisms of dispersal and sowing. Unusual growth-forms are particularly prominent on islands (https://www.sciencedirect.com/science/article/pii/S0960982220313506 and https://pubmed.ncbi.nlm.nih.gov/33007244/) and at high altitudes (https://en.wikipedia.org/wiki/Dendrosenecio).
Yet, this adaptive radiation seems to have been surprisingly reluctant to produce something common in many other dicotyledonous families, large and small: hard leaves (https://en.wikipedia.org/wiki/Sclerophyll).
There is also a conservatism in the floral forms in Asteraceae (https://en.wikipedia.org/wiki/Asteraceae#Inflorescences). Daisies typically possess a pseudanthium (https://en.wikipedia.org/wiki/Pseudanthium), in the form of a capitulum. Pollination is by insects, with few exceptions (https://onlinelibrary.wiley.com/doi/abs/10.1111/plb.13283#:~:text=Abstract,been%20suggested%20for%20some%20species. and https://www.researchgate.net/post/Bird_pollinated_Asteraceae and https://www.britannica.com/plant/Asteraceae/Pollination and https://www.jstor.org/stable/2444398).
The floral conservatism in Asteraceae is less surprising than the foliar conservatism. This is because plant families are generally defined mainly by their flowers.
The leaves of daisies vary greatly in size, shape, and seasonal behaviour. However, extremely few species are sclerophyllous. i.e. possess lignified leaves with cardboard- or plastic-like textures. This seems particularly odd in view of the general affinity of daisies for dry climates and open vegetation,
So, it is worth noting that a peculiarity of daisies is that it has many foliar-spinescent taxa but few sclerophyllous taxa. I do not recall seeing this point made in the literature. The typical leaf of xeromorphic daisies, such as those common in mediterranean-type climates, has a felty, pale underside different in appearance to the upperside of the leaf.
Foliar spinescence is relevant to this topic, because anti-herbivore spines on leaves generally depend on the rigidity of the leaves.
As far as I know, there is no member of the Asteraceae with the kind of foliar spinescence common in Proteaceae, Ericaceae, and Fabaceae. Because foliar spinescence is partly contingent on scerophylly, it makes sense that few spp. of daisies have spinescent leaves of the sorts so common in kwongan.
The closest to this is Olearia pinifolia (https://en.wikipedia.org/wiki/Olearia_pinifolia), a shrub restricted to kwongan-like vegetation in Tasmania. However, even in this case, the rigidity of the needle-like leaves is owing to the rolling of the leaf-blade, as much as the lignification of the tissues themselves.
Spinescent leaves are extremely common in daisies, but mainly in the category of ‘thistle’, which configures foliar spines on non-sclerophyllous foliage.
The usual way in which daisies (other than the many taxa of thistles) defend their foliage is chemically, including by means of aromatic, resinous or gummy substances.
Among plants in general, there is something of an inverse colouration between chemical defence against herbivores and foliar spinescence. Given the emphasis by daisies on chemical defence, it makes sense that few of the non-thistle taxa extend to a combination of sclerophylly and foliar spinescence. Even the apparently holly-leafed members of the Australasian genus Olearia hardly qualify as foliar-spinescent, because the ‘teeth’ on their leaves tend not to be ‘pungent’.
An interesting biogeographical pattern is that perennial daisies tend to be uncommon in kwongan, the vegetation which has the greatest incidence (i.e. combination of commonness and spp.-diversity) of both sclerophylly and foliar-spinescence, worldwide. Even compared with fynbos in South Africa, daisies are noticeably scarce in kwongan, wallum, and dry sclerophyll forest, and those few spp. that do occur tend to have relatively soft leaves.
Given the above background, it is interesting to find several spp. of shrubby, small-leafed (ericoid) daisies in fynbos in South Africa that do qualify as foliar-spinescent. These have no counterparts in Australia or New Zealand as far as I know, even in the genus Olearia.
The genus Oedera is nearly restricted to fynbos, and contains 17 spp. Of these apparently only four spp. are foliar-spinescent; they are semi-sclerophyllous, but their leaves remain small, i.e. comparable with those of ericas in Australia, rather than proteas and peas in Australia.
As expected for ericoid daisies, most of the spp. of Oedera are chemically defended, and there seems to be a pattern in which the chemically defended spp. and the foliar-spinescent spp. tend to be mutually exclusive.
Oedera capensis
http://fernkloof.org.za/index.php/all-plants/plant-families/item/oedera-capensis and https://keyserver.lucidcentral.org/key-server/data/04040f0a-0603-4405-880c-010703060006/media/Html/Oedera_capensis.htm
Oedera imbricata
http://www.fernkloof.org.za/index.php/all-plants/monthly-flowers/item/oedera-imbricata and https://keyserver.lucidcentral.org/key-server/data/04040f0a-0603-4405-880c-010703060006/media/Html/Oedera_imbricata.htm
Oedera laevis
https://keyserver.lucidcentral.org/key-server/data/04040f0a-0603-4405-880c-010703060006/media/Html/Oedera_laevis.htm
Oedera steyniae
https://keyserver.lucidcentral.org/key-server/data/04040f0a-0603-4405-880c-010703060006/media/Html/Oedera_steyniae.htm
(writing in progress)
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