Spinescence in Ceratozamia, with a summary of all the American cycad genera
In other Posts I have summarised the configurations of spinescence in two of the five American genera of cycads, namely Dioon and Zamia.
Here I complete the picture by describing the configuration of spinescence in Ceratozamia (https://www.inaturalist.org/observations?place_id=any&taxon_id=129813&view=species), Microcycas (https://www.inaturalist.org/taxa/135860-Microcycas-calocoma), and Chigua (https://www.inaturalist.org/observations?place_id=any&taxon_id=67326&view=species), which are also restricted to tropical America.
My source is Jones (2002, https://www.amazon.com/Cycads-World-Ancient-Plants-Landscape/dp/1588340430 and https://books.google.com.au/books/about/Cycads_of_the_World.html?id=lFEeAAAACAAJ&redir_esc=y).
Ceratozamia contains 16 spp., mainly in Mexico.
Most spp. of Ceratozamia have prickles on the petiole, and a few have prickles also on the rachis. The petiolar prickles are reduced in some spp. and absent in a few. In at least one sp. these petiolar prickles are ‘curved’. Only one sp. (C. sabatoi, https://www.inaturalist.org/taxa/136167-Ceratozamia-sabatoi) has a spine (’pungent’, https://www.finedictionary.com/pungent) at the tip of each leaflet, as well as both petiolar and rachis spines, making it the most spinescent species in the genus. Several spp., living mainly in rainforest, are not sclerophyllous (https://en.wikipedia.org/wiki/Sclerophyll).
Microcycas contains one species, in Cuba. It is non-spinescent.
Chigua contains 2 spp., in Colombia. The configuration of spines is so similar to that seen in rainforest spp. of Zamia that we can consider this genus effectively included in Zamia w.r.t. spinescence.
Dioon is the American genus most similar to African Encephalartos in its configuration of spinescence. The petioles are prickly only in one species, which also has spines on the tips and margins of the pinnae. The tips and margins of the pinnae are spinescent in four of the 12 spp. in the genus, while only the tips of the pinnae are spinescent in 2 spp. This genus is the least associated with tropical rainforest of the American genera, instead occurring in relatively dry areas and open vegetation.
I have shown, in another Post, that in Zamia there are often petiolar (and rachis) spines, but there is no spinescence at the tips (or probably on the margins) of the pinnae.
As far as I can see, not one of the many American spp. of cycads possesses spinescence on the proximal, reduced pinna, a pattern so common in both Australian Macrozamia and African Encephalartos. However, in a way the petiolar and rachis spinescence is a functional substitute for this.
What this means is that we can arrange the American genera as follows.
The complex of genera, Zamia/Chigua/Microcycas, which is associated mainly with tropical rainforest, is often spineless; those spp. which are spinescent have spinescence only on the petiole (and in some cases rachis).
I have come to see this as a ‘tropical rainforest pattern’ in cycads, associated with thin, papery (non-sclerophyllous) pinnae.
It seems unremarkable that even cycads lose their sclerophylly in tropical rainforest. However, it is significant that many spp. retain spinescence in the form of petiolar spinescence (extending in some cases to the rachis).
This suggests that, if the spinescence is adaptive anti-herbivory, the relevant herbivores tend to grasp the leaves by the base (petiole) and either pull them off the caudex or strip them, upwards, of their leaflets. Petiolar spinescence would seem to retard the rate at which large herbivores could ‘pluck’ the leaves of these mainly rainforest cycads, the pinnae of which are not lignified enough to constitute excessively fibrous food for herbivores. And many spp. in this complex are completely non-spinescent.
In the genus Ceratozamia, only one species is non-spinescent; and one species has pinna-tip spinescence in addition to the usual petiolar spinescence. This means that Ceratozamia is hardly more spinescent than the complex of genera described above.
In the case of the genus Dioon, the configuration has shifted significantly in accordance with foliage that is far stiffer/more rigid, and more sclerophyllous, than in the above genera. Only one species possesses petiolar spinescence, while both pinna-tip and pinna-margin spinescence occur in one-third of the spp. In this genus there is a significant convergence with African Encephalartos in the configuration of spinescence, which makes sense because Encephalartos does not occur in tropical rainforest.
One of the main facts I have learned from my perusal of Jones (2002) is that cycads associated with tropical rainforest tend to be non-sclerophyllous, with spinescence restricted to the petiole, sometimes extended on to the rachis.
Since herbivores are unlikely to try to eat the petiole or rachis itself, this configuration of spinescence seems designed to prevent herbivores from grasping the leaf at its base and pulling, which could reduce the foliage rapidly to the detriment of the plant. Such a configuration is not in itself puzzling; it is more the contrast with typically Australian and African cycads, familiar to naturalists living in extratropical regions, that is remarkable.
Cycads in the Americas differ from those in Australia or southern Africa in failing to extend to temperate latitudes (> about 30 degrees). In the case of South America, cycads are puzzlingly absent everywhere south of northern Bolivia. This help to explain the intercontinental differences.
However, it remains true that e.g. Zamia integrifolia (https://www.inaturalist.org/taxa/135560-Zamia-integrifolia), which reaches southern Georgia in the USA, is puzzlingly non-spinescent.
תגובות
הוספת תגובה