Comparison of the cycads Zamia and Macrozamia
If one lives in Australia or South Africa, as I have done for most of my life, one tends to get the impression that cycads have tough, plastic-like leaves. If one pays attention to details, one also gets the impression that these tough leaves have spinescent pinnae. Thus one forms a certain impression of ‘the typical cycad’, and this impression is of a leaf-spinescent plant.
In this Post, I focus on a tropical American genus of cycads, namely Zamia, which contains many (53) species. The point is to show that there are plenty of cycads in the world that have leaves of ‘normal’ texture, lacking spines on the pinnae. These spp. generally live in rainforests, free from fire.
And in this genus, Zamia, the ecological versatility of cycads is so great that at least one species qualifies as an epiphyte, and another as a denizen of mangroves.
My source is D L Jones (2002). This author provides the species-descriptions but makes no attempt to synthesise the intergeneric comparisons in the way I do here.
Please bear in mind that in terms of habitat the two genera have different emphases. Zamia typically occurs in tropical rainforest, although some species occur in savannas. By contrast, Macrozamia typically occurs in various types of relatively open, fire-prone vegetation at higher latitudes, and never penetrates tropical rainforest, let alone equatorial rainforest.
However, it seems fair to compare Zamia living in Florida with Macrozamia living in southeastern Queensland.
Although most spp. of Zamia have pointed pinnae with small teeth (which Jones describes in some cases as ‘spiny’), neither the tip of the pinna (which is never described as ‘pungent’ in this genus) nor the teeth on its margin seem to be able to hurt the human hand. What this means is that in the great majority of spp. the pinna is not, in fact, spinescent (although I cannot rule out some sharpness similar to that of ‘saw-edged’-sedges).
Although Zamia lacks leaf-spinescence on the leaf-blades themselves, many spp. of Zamia do possess a different kind of leaf-spinescence:
- prickles on the rhachis in some spp., and
- prickles on the petiole in most spp.
In e.g. Zamia chigua, the prickles on the petiole are described as including ‘branched’ prickles, whatever that means.
Overall, the ‘typical’ species in genus Zamia has a spinescent petiole but non-spinescent pinnae.
In this way, Zamia can be seen as the ‘opposite’ to the genus Macrozamia of Australia. This is because Zamia
- tends to have spinescent petioles whereas Macrozamia never does,
- has the petiolar prickles in some cases extending on to the rhachis, whereas no sp. of Macrozamia has spines on the rhachis,
- lacks the spiny tip to the pinna which is so common in Macrozamia, and
- often has ‘teeth’ on the margins of its pinnae, which Macrozamia lacks, although neither genus has spinescent margins to the pinnae.
Although certain spp. of Zamia are sclerophyllous (leaves described by Jones as ‘thick and leathery, stiff, rigid’ in the case of e.g. the horticulturally popular Z. furfuracea), other spp. of Zamia have leaves so thin that the leaves of Z. hymenophyllidia are described as ‘membranous’! What is particularly significant about this is that even in this membranous-leafed sp. the petioles retain ‘very small prickles’.
What this indicates is that petiolar spinescence is adaptively ‘decoupled’ from pinnal spinescence, not so? Petiolar spinescence lacks a relationship with sclerophylly, and this is as true in cycads as it is in plants generally.
Zamia integrifolia is worth mentioning specifically, because it shows that even a species adapted to open vegetation in relatively dry climates remains non-spinescent. Zamia integrifolia is widely distributed from southeastern Georgia in the USA to certain Caribbean islands and the Bahamas. It was once abundant in Florida, where its stems were exploited for edible starch by the Seminole native Americans, just as the same species was exploited in the Caribbean by the Arawak native Americans.
Despite being so subject to damage, despite growing in fire-prone vegetation, and despite having ‘stiff, leathery’ pinnae, Z. integrifolia is non-spinescent. Its petioles lack prickles. Its rhachis accordingly lacks prickles. The pinna, far from being ‘pungent’, has that rare thing for a cycad, a blunt apex! And even the marginal ‘teeth’, so common in Zamia, are in this species small and blunt, described as mere bumps, i.e. far from spinescent.
At a casual glance, Zamia integrifolia, which is regarded as hardy and adaptable in horticulture, looks much like the typical form of Macrozamia (Australia) or Encephalartos (Africa). However, on closer inspection it proves to be lacking in any spines. And in this case this cannot be explained by an association with rainforest.
The most epiphytic species of Zamia, Z. pseudoparastica, has a few prickles on its petiole, a non-spinescent (non-prickly) rhachis, and non-spinescent pinnae. Intriguingly, the diaspores of this specialised cycad are aberrant in having the sarcotesta sticky-mucilaginous and with a distinctive sour smell, presumably in aid of dispersal by fruit-eating bats and birds. Here we see convergence with mistletoes in the mutualism with vertebrates in aid of dispersal and sowing in suitable sites, i.e. on branches rather than on the ground.
At least one species, Zamia roezlii, occurs in mangroves, where it may be flooded by high tides. And this species, occurring in e.g. Ecuador, is also non-spinescent.
I assume that Zamia, which is known to be toxic, relies on toxins to the exclusion of leaf-spinescence in its pinnae. However, I have yet to explain why Zamia would opt for spinescence on its petioles.
I can summarise these differences as follows:
In both the Australian genus Macrozamia and the American genus Zamia, some spp. lack any spinescence in their leaves. However, those spp. which are leaf-spinescent differ in a basic way between the two genera. Macrozamia defends its leaves from herbivores by ‘marginal’ spinescence, whereas Zamia defends its leaves from herbivores by ‘basal’ spinescence.
In Macrozamia, the spinescence is distal; in Zamia, proximal. I.e. in Macrozamia the spinescence is at the tips of the pinnae (and not on the petioles), a pattern consistent with sclerophylly.
By contrast, in Zamia the spinescence is on the petioles, not at the tips of the pinnae. This is instead consistent with non-sclerophylly, i.e. with a flimsy (papery rather than leathery) texture to the leaves.
I find it surprising that these major groups of cycads take such different approaches to anti-herbivore defence. It almost seems that the pressures of herbivory in equatorial rainforests tend to be qualitatively different from those in the various kinds of open vegetation in temperate-zone Australia.
And the notion that cycads are anachronistic ‘dinosaur plants’ is obviously false, in view of the versatility of cycads even in rainforests.
ILLUSTRATIONS
Zamia 'maritima', showing the prickles on the petiole.
https://growerjim.blogspot.com/2017/02/zamia-maritima-cardboard-plant.html
Zamia integrifolia, which looks superficially like Macrozamia but is in fact non-spinescent, lacking spines even on the petiole.
https://garden.org/plants/photo/555455/ and https://tropical.theferns.info/viewtropical.php?id=Zamia+integrifolia and https://florida.plantatlas.usf.edu/Photo.aspx?id=19723 and https://en.wikipedia.org/wiki/Zamia_integrifolia
Zamia furfuracea, showing the prickles on the petiole. This species has sclerophyllous pinnae but the pinnae are not spinescent. Instead, it is the petiole which is spinescent.
https://austinbotany.wordpress.com/2015/03/14/zamia-furfuracea-cardboard-palm/ and https://www.baobabs.com/Fiche2.php?Lang=en&Ref=582 and https://www.florida-palm-trees.com/cardboard-palm-tree/ and https://en.wikipedia.org/wiki/Zamia_furfuracea
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