Wednesday, April 15, 2015

Dichotomous key (VII.d) - The world of internodes cont'd (Adornment)

Adornment - thorny parts
A twig at its most basic form, is rather helpless against predators. Fortunately, trees do not leave them unaided. For example, sap can contain a range of chemical defenses (like the toxic orange sap of staghorn sumac) - this is particularly prominent later in spring when the sap gets, as they say in the sugaring world, buddy; I've written about that previously. Many plants, however, have figured out clever solutions by remolding different parts to serve a more spiky purpose; we only have a scant few trees in our area that have these spikes along their stems - though there are a number of delightfully delicious plants that have thorns (e.g. raspberry, blackberry, roses). These sharp projections derive from three different parts of the plant, the stem, leaves, or epidermis. Also good to remember that the terms below are botanical terms and are vernacularly often used interchangeably.

Chickadee flies from buckthorn branch
Spines are projections that form from the stem of the tree, arising from buds. In buckthorn, as shown above, they form at the tips of branches, or rather these spines are the tips of the branch. Hawthorn has very prevalent spines, as do honey locust and osage orange.

Thorns are modified stipules or leaves. The sharp projects on barberry and black locust, for example, are thorns. Above are the paired thorns of black locust, or pointed ears of Marrowzodufia Blugly the Dwarf, as you'll remember from my posting on bud scars.

Multi-flora rose thorn
Blackberry thorn with a scar just above it from where another thorn fell off
Prickles derive from the epidermis and are not vascularized (hence the can be popped off quite easily). Roses, raspberries, and blackberries all have prickles. Tracing the stem of a blackberry, rose, or raspberry, it is easy to find a number of scars dotting the stem from where thorns have fallen (or been ripped) off.

Tendril anchoring a grape vine to another twig. 

Showing adhesive pads on the ends of Virginia creeper
Adornment - clasping parts
Since vines put very little investment into their stem for structural support, and therefore need some mechanism by which they can cling to other objects as they wend their way up to the top of the canopy. Grape vines and Virginia creeper (both in the grape family, Vitaceae), have tendrils, which are highly specialized anatomical features for vining, originating from the stem. Virginia creeper, which is in the grape family Vitaceae, even has adhesive pads at the tips of the tendrils. 

Zoomed in view of a sharp and grippy bud of a bittersweet vine
Bittersweet climbing a sugar maple sapling
Tumurous growth of a sugar maple in response to a bittersweet vine
Other vines, like hardy kiwis and oriental bittersweet, accomplish this task with pointy buds that anchor the vine as it spirals and wraps around a substrate, often strangling and killing its unwilling host over several years. The constricting force around the stem of the tree being strangled cuts of circulation in new conductive tissue, effectively cutting off the host's supply chain. We also have a number of perennial herbaceous vines that encircle the substrate as they climb up it (e.g. deadly nightshade, morning glory, bindweeds, hops).

Poison ivy vine attached to white pine trunk. Image is from August 2012
Poison ivy vines have thousands of root hairs that anchor the stem to a surface. No other vine in our woods have this. This paper provides a good synthesis of different strategies of vining plants and unanswered questions concerning vines.

No comments:

Post a Comment