The Science of Botanical Art

Climbing Plants

Dick Rauh

Originally appeared in The Botanical Artist - Volume 14, Issue 1

 

The climbing or vining habit is a wonderful example of the economy of nature. It allows a plant to attain maximum exposure to sunlight, water and nutrients with a minimum expense in vegetative support. Why spend all that energy in making strong stems when you can use existing sturdier plants as steppingstones to attain the same goals? This is probably one of the reasons why so many of our nastier invasive species fall into this category, and are so successful. Look at the honeysuckles Lonicera spp and bittersweets Celastrus spp for example. Most of the time the climbing habit can be contained and managed with trellises and judicious pruning. Unconstrained vining growth can be a major environmental threat. It so often happens that the vine growth destroying the host plants is an invasive species, and what are destroyed are native trees and shrubs.

Vining, it can be argued, is a late adaptation on the evolutionary time line. There’s no sense in climbing, unless there exist vertical plants on which to climb. Besides these are the plants whose height and arching canopy stole the access to light that created the need to find a way out of the shadows in the first place. 

The solution to this problem makes use of methods of growth always present in the genetic make-up of plants, and which can be traced back to the earliest land plants that existed. Common to most climbers are rapid growing, elongated axes with suppressed lateral growth, hence weak stems. Circular plant movement, always present in a minor way, now becomes a method for those seeking support to probe the environment for something to climb on. Darwin named this process circumnutation. I have seen time-lapse studies of the searching movement of some vining plants and the extent and purposefulness of the action as they grope in ever widening circles is truly amazing, if not a little spooky. 

The mechanics of vining involve a number of techniques to serve the purpose, and it is those that appeal to us as artists. Adrian Bell, an Australian morphologist, breaks them down into four classes – scramblers, adventitious root climbers, tendril and hook climbers and twiners. Scramblers are most likely the most primitive. These are the plants that just sort of sprawl over nearby vegetation. They make the most of their inherently weak stems, and have developed thorns or pickles or other sticky devices to help them. Think of Rosa multiflora as a prime example (and another successful invasive species). 

The ability to succeed as vines is based on the existence in these plants of a cellular sensitivity to touch and darkness that triggers complex physiological processes. It is these processes that lead to the mechanical adaptations that serve this purpose in different ways. Plants, like ivy Hedera spp and poison ivy Rhus toxicodendron have developed short clinging roots all along the stem to help them attach to bark and bricks and other rough vertical surfaces that lend themselves to this type of invidious attachment. These are not true roots in the traditional sense, but probably arose from the rhizoids of certain sprawling primitive mosses. 

Next we get to the use of tendrils and other hook-like devices. The cross section of a tendril tends to be flattened, and what makes it curl is the uneven cell growth of the opposite sides. It would form a spiral, if growth along the long axis didn’t transform it into the corkscrew we all know and love. The rotation of these twists varies with different species depending on whether it is the top or bottom side of the stalk that is sensitive. Tendrils will grow to full length and use circumnutation until they come into contact with a likely support. Often you will find a tendril with both clockwise and counterclockwise spirals spaced by a loop. This is generally because the tendril has attached to a host and is in the process of tightening up its grip. If they don’t make it, most will wither and fall off; some will roll-up to try another time.

Now it gets kind of interesting, because the origins of tendrils vary in different plants. Take greenbrier Smilax (you take it, I hate the darn stuff), where the tendrils appear to be derived from stipules and arise from either side of the leaf petiole. These don’t appear to do much twisting, until they find a host to hang on to. Then there are tendrils that are modified leaves. A loss of lamina tissue allows the midrib to take on the grasping function as in the case of many members of the pea family, Pisum sativum for example. The tendrils of grape Vitis, are modified shoots. It is the apical shoot that develops into the single tendril across from the leaf or inflorescence, and the lateral shoot that is stimulated to continue the stem growth. In some instances the tendril is compound, the single tendril forming a number of branches as in Virginia creeper (Parthenocissus). Take a close look at these branches. You will find a small suction cup at the end of each. 

Twiners are plants where the shoot itself acts as the climbing mechanism. Here is where the rotation is clockwise in the northern hemisphere, not because the stem has some inherent mechanism to guide it, but because as it grows it follows the path of the sun from east to west. It probably needs to be said, that once the initial purpose of these devices is achieved there are morphological and physiological changes that occur, primarily for us a visible hardening of the tissues.

There are some perennial climbing plants that have managed to take this hardening process a step further. These make use of the economies of vining, and then take on the strength and support based on secondary growth. These are the so-called woody vines or lianas. Locally Wisteria and Celastrus have stalks that have bark as a sign of cambial growth. However lianas are largely tropical where dependence on weak stems would prove a disadvantage in the competition in the canopy. Besides, can you see Tarzan swinging from tree to tree on a skinny garden pea?

  • Tendril anchored vines, pen and ink, ©Dick Rauh