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Callus - Callus is a soft, non-woody tissue that forms about the edges of fresh wounds. Callus is undifferentiated, meristematic tissue, which has very little lignin. After several weeks to a few months, during which period, it is then replaced by woundwood. After wounding, callus forms first about the margins of the wound; Woundwood forms later as the cells become lignified. It is important not to confuse “Callus” with “Callose” and or “Callous”.

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Callus and Woundwood. Callus is a tissue that is meristematic. That means it has the capacity to divide and differentiate and to form sprouts, roots, leaves, buds. Meristematic – the capacity, the capacity, the capacity. With callus, the cells are all round, they have very little lignin, they all look the same. As they are released, the meristematic cells in the wood that are surrounded by the apoplast, now have an opportunity, to divide and they divide and become nice round cells. They are meristematic. Woundwood doesn’t happen immediately. See many people do not understand this term. Dr Shigo did not do this. This was the work of Ernest Kuster in 1903. As cells are released by wounds, pruning, bashing into the tree, the meristematic parenchyma cells that are trapped and held in place by the apoplast now have an opportunity (i.e., those that survive after the wound and are still able to divide and differentiate) to divide and as they divide they become nice round cells. You see the wound has released pressure. It has released these meristematic living cells in wood and bark. The cells then have very little lignin and they spent all of their money or all of their energy having more kids – more cells. However, as the cells divide and divide and divide and have division parties – they all get together and we are all going to divide, the pressure begins to come back. Some cells say, its my turn to divide, no its my turn to divide, so one says if I can’t divide and become more big round cells I might as well just become a vessel, a fiber or maybe I’ll become a parenchyma cell and just maintain my living content. We do not know how it all goes. We do not know, we do not know, we do not know. But we do know there comes a time when division begins to go in a way we just described. Then these cells begin to loose their meristematic ability – some of them, not all of them. And they begin to cash in if you wish, use if you wish, utilize (we do not know) some of their energy to form lignin. Lignin is extremely expensive material. And then they become, in time, not callus but woundwood. People find it very difficult to understand this. People too often admit in writing that they do not understand these processes by misusing the words callus and woundwood. They say we always used that word, which shows they are not that bright, and they want to maintain that word and use it. See callus does not usually close large wounds. Callus may close a very small wound, but most of the time its woundwood, its wood. So as the arrangement of the cells begins to come back to its normal state then we go to wood. So we start with normal wood and it is wounded, released, some still alive. Those that are still alive will then produce callus. As the callus, as the pressure returns, some of the cells begin to become woundwood.
Or if you want to think of it this way, maybe you will understand it better this way. Here we have in the beginning, all are happy in their orderly arrangement. Then comes the wound. Those that can survive this injury then begin to reproduce or have new cells – this is callus. Lets say that is 100. And then some begin to leave the group and become as they were before and then its 99-1. Then its 90-10. Then 80-20. Then 70-30. Then 50-50. Then 40-60. Now we are going toward woundwood. Because you see callus does not suddenly become woundwood, it takes time. That is why these two words are so difficult to understand. Please let us help you. Please make certain you use the correct words. If you don’t use the correct words then we don’t know what you are talking about. Then we have gibberish, confusion and then an argument. Please use the correct words. Think, think, think.