Wednesday, 3 October 2012

Cambium - The Living Part of a Tree or Shrub

Diagnosing Death

Because a plant looks dead doesn't necessarily mean that it is. Woody plants - trees and shrubs - can often display all the symptoms but not actually be in the same realm as the Monty Python parrot: the living, growing, part of a tree trunk is immediately under the bark and is called the cambium. It is green, which is a diagnostic feature to look for if in any doubt. Late spring and early summer are the times when most healthy plants are mistakenly consigned to the compost heap. Because it is late coming into leaf it is easy to think that the winter has put paid to it and the only way is out. I don't fall for this one any more. Simply by scraping a thumbnail of bark back will show whether the cambium is still living and green, or brown and dead. If it is green, then a bit more patience is called for, giving it time to belatedly get out of bed.

I was surprised to learn that hamster death symptoms are similarly not straightforward. My son was in a house- share when one of the sharers - a girl - came banging on his door in floods of tears. It seemed her hamster had died and he, known to be compassionate where young women are concerned, was designated as undertaker. He duly wrapped the deceased carefully in kitchen foil, descended to the yard, and consigned it to the wheely-bin. While this sad ceremony was taking place, the bereaved flatmate was tearfully informing her boyfriend of the death over the phone. It turned out that the boyfriend was a bit of a wildlife expert: "it isn't necessarily dead," he informed her, "they hibernate. Warm him up a bit and he'll probably wake up." This learned information was immediately imparted to him when he returned from the interment and he was promptly dispatched back to the yard to perform an exhumation. The animal was then placed gently under the grill on a very low setting. Unfortunately, Jesus he was not, and he remained in the same state as the aforesaid parrot.

Hollow oak at Lingfield, Surrey

Anyway, cambium consists of the tree's arteries: water, nutrients and sugars are pumped up and down through them and each year they die and new ones are formed under the bark as replacements. This means that the centre of the trunk is dead material, it also explains how the rings seen in a cut trunk have been made: at the beginning of the growing season wide arteries are formed to accommodate the rush of water, then as the summer progresses, growing drier (theoretically), smaller ones replace them, creating the different textures which appear as rings. Counting them will give the age of the plant. The appearance of the rings can also give an idea of the amount of rainfall in a particular year: one tree, a bristlecone pine living in The White Mountains of California, displayed 1,100 microscopic rings in 5 inches, mirroring the dryness of the area where it lived.

Hollow, live tree in Dunham Park

Knowing about the cambium is useful not only in determining whether a plant is alive. French walnut growers would, after a bad harvest, beat the tree with willow wands. This would cause neighbours to think about calling for the men in white coats but, amazingly, the following year's crop would be abundant. The reason for this was that beating damaged parts of the cambium, limiting the flow of growth-promoting sugars to the roots. This meant that sugar, continually produced in the leaves, had now to be re-routed somewhere and the only option was  to the upper parts of the tree. Here it was used up in promoting more flowers and fruit. We now have a more sophisticated way of doing this by bark ringing. This is the removal of a strip of bark, about an inch wide, from perhaps three quarters of the trunk's diameter. Beware though, if you ever try this, completely circling the tree will lead to it being in the same state as the hamster.

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