Acacia denticulosa as an extreme species among foliar-spinescent wattles

(writing in progress)
 
Among phyllodinous spp. of Acacia in Australia, most species are not ‘leaf’-spinescent. Those relatively few spp. of Acacia that do qualify as foliar-spinescent tend to have small phyllodes, perhaps because small phyllodes can more easily provide the rigidity required to make foliar spines effective.
 
I do recall, offhand, any species of Acacia which has a typical ‘holly-leaf’ shape in the phyllode.
 
However, one species is noteworthy because it manages to be marginally leaf-spinescent with an unusually large phyllode: Acacia denticulosa (https://apps.lucidcentral.org/wattle/text/entities/acacia_denticulosa.htm and http://www.worldwidewattle.com/speciesgallery/denticulosa.php?id=3293 and https://en.wikipedia.org/wiki/Acacia_denticulosa).
 
The leaf-spines of Acacia denticulosa are not exactly painful to the touch because they would not be described as ‘pungent’. However, the plant would certainly be described as ‘prickly’. And what is most interesting is how the most is made out of the (limited) spinescence on the phyllode-margin. by the peculiarly curled shape of the phyllode, a structure that is three-dimensional instead of two-dimensional as in most spp. of phyllodinous Acacia.
 
The phyllode is extremely sclerophyllous although not particularly thick. Although a phyllode is by definition a modified phyllode, this phyllode has a short petiole of its own (https://www.agefotostock.com/age/en/details-photo/flower-raceme-of-the-endangered-western-australian-native-tree-acacia-denticulosa/GWG-RMC256).

The phyllode has a rough (‘sandpapery’) surface. Its margins have numerous prickles, so that the phyllode is ‘abrasive’ to the touch regardless of how it is grasped. And most noteworthy is the fact that there are two types of ‘contortion’ in the phyllode:

• the margins are wavy, which ‘deploys’ the marginal spines in three dimensions, and
• the whole lamina of the phyllode is curled.

The phyllode is so stiff that it is impossible to ‘straighten’ the phyllode, i.e. to lay it out flat. If one forces it, the phyllode, which is not brittle, just buckles in another way. As a result the plucked phyllode is difficult to put in one’s pocket because it has such an awkward shape with prickles that catch on to cloth at every turn.
 
In what has been written in the scientific literature on leaf-spinescence, I am not sure that enough mention has been made of a certain aspect of the ‘holly-leaf’ kind of spinescence: that it tends to make the leaf not only harmful to the touch but also difficult to grasp from any angle because the spines are variously pointed in three dimensions. I.e. it seems basic to the phenomenon of foliar spinescence, where there are more than one spine per leaf, that the leaf is modified to be as three-dimensional as possible.

And here again, the ligneousness (sclerophylly) of spinescent leaves aids this effect because the leaf can be rigid enough to maintain its undulations/waves/curls against pressure by an animal.

(writing in progress)

הועלה ב-יולי 4, 2022 12:29 לפנה"צ על ידי milewski