Olpea, some of the wood is xylem, but as the tree grows the interior xylem dies and becomes heartwood with very few living cells (the living cells are like the fungus police). The older the tree the lower the percentage of the wood is xylem (sapwood).
http://northernwoodlands.org/articles/article/what_is_the_difference_between_sapwood_and_heartwood
yeah, really strange that available literature always mention how urgent it is for scion cambium be in immediate contact with rootstock’s cambium, but not sure if any has even mentioned the most critical aspect of grafting–that is for scion sapwood be in direct contact(with minimal air-gaps) with the rootstock’s sapwood.
while it is true that cambium contact is beneficial when grafting, contact of rootstock xylem to scion xylem is at least as vital, especially during spring when net nutrient flow is temporarily in reverse direction(from rootstock to scion–think maple syrup). And of course, at least as important is supply of moisture, which is a perpetual year-round thing from rootstock to scions.
girdling trees actually trivializes “the urgent need for immediate cambium contact”, as the rootstock will actually continue to support all stems/grafts above the girdle, and in fact, all stems/grafts above the girdle will fatten up as the downward flow of sugars and proteins has been permanently severed(phloem).
once the food depots within the roots have been consumed, the rootstock dies first, and only then will the above-the-girdle stems die. Thus said, if scions fail outright on the year of grafting it is mainly because upward sapflow was compromised or lacking, and not because of poor cambium contact, as it is obvious from the girdled elm scenario that zero cambium contact does not kill above-the-girdle stems or grafts.
so going back to the girdled elm trees which were never allowed to have cambiums re-connect: the stems above the girdle were ok, and in fact selfishly accumulating all the nutrients produced via photosynthesis for two years. Meanwhile, the roots were slowly dying of starvation which took two years to consume their unreplenished food depot.
perhaps important to note that roots and stems keep their food depot of sugars mainly in condensed starch form(apart from wood cellulose). My personal analysis on this(maybe am wrong on this one, as haven;t come across literature that mentions this-- but also haven’t come across one that says otherwise)is that come spring, this stored starch is hydrolyzed into simple sugars(which nodes/buds need as food to leaf out), and these sugars in solution exert stronger osmotic/hygroscopic properties than regular starch* or wood, sucking in moisture from the soil, and resulting in an increase in internal volume of fluids. This increase in volume is likely one of main reasons why sweet-sugary maple sap flows upwards.
*if you enjoy baking, then very much aware that cake is mainly starch(which is a complex chain of sugars). And if you want your cake moist for longer periods you have to load it with simple sugars, say, sucrose and/or fructose, as simple sugars have a higher affinity for ambient humidity than starch
i disagree.
The xylem is no longer actively dividing(it’s mostly dead), and thus once cut can not fuse or merge with other xylem. And thus cannot form a vascular system.
When grafting the “pay dirt” is the lateral meristem. This is the layer of “stem” cells that divide. These stems cells then differentiate into different vascular systems (xylem towards the inside of the stem. containing upwards flow from the roots) and phloem towards the bark (outside of stem) the phloem transports both up and down, and can be seen more as a redistribution system of tree resources (sugars hormones etc) but is also responsible for delivering resources to the roots of the tree. The phloem is a more “active” (“pumping” in directions) system. where the xylem is more passive (relying on pressure from the roots and negative pressure from evaporation in the leaves.
Think of the xylem as long continuous straws. (mostly dead tissue)
With flow only going upwards
And the phloem as “tubes” connected to pumps. With flow going both ways. (even adjacent “tubes” can flow in different directions)
The lateral meristem (grafters “pay dirt”) is located in the vascular cambium.
There is however not a super clear line between the lateral meristem and the phloem/xylem. Since the cells from the meristem are in different phases of differentiation to the phloem and xylem.
Anyway the important thing for grafting is, that you get the lateral meristem of the scion and rootstock close enough. So that the exces undifferentiated cells they will produce when damaged (cut) in the form of callous can touch, and then fuse together. And when fused together can differentiate into vascular tissue and thus make a vascular link between stock and scion.
The closer the meristem of the scion is to the stock, the less gap it has to fill with undifferentiated cells. And thus the faster it fills that gap, and the faster there is a connection to initiate the differentiation of the cells to make a vascular connection.
And a faster connection gives the scion less time to run out of resources (water and nutrients, but mainly water)
Another important factor is to not damage/kill to much of the meristem. Since then there is “dead” debris between the 2 meristems trying to fuse. This happens for example when you cut a scion and than let it dry out before pushing the cut surface to the stock.
Or when you coat the cut parts of your graft with a fatty film. (eating chips and then touching the cut surface? or using a oil covered knife)
Back to the original question of the topic.
I think the cambium itself is to small (few cells thick) to actually see with the naked eye. While grafting we aim for where the meristem cells differentiate into the phloem (the dark (green) line that’s visible) On the inside of that line we find the vascular cambium/lateral meristem. As far as I’m aware the cambium is reasonable translucent. And basically has the colour of the dye it absorbs in microscopic pictures. The cells in the meristem are smaller though. And increase in size when the differentiate/elongate.
It might have a slight tint, but due to it being so thin, a small coating of cambium cells on the xylem for example will likely not give a visual colour change.
i suspect the colour to be different for example on B9 (redder translucent) than on M9. (translucent or green tint)
Anyway the exact colour or spot of the cambium does not matter much. Since it’s so thin you could never match it 100%. You just want to aim for “close enough” and let the callous make the connection.
when grafting, (in chronological order)
we cut/damage the scion and stock. On the cut surface of the lateral meristem, callous tissue (undifferentiated cells) will start to form excessively. Until they have bridged the gap between scion and stock.
Once this happens these cells will start to differentiate into vascular tissue’s.
On the inside (towards the wood) it will form xylem tissue (mostly dead long straws that only transport upwards)
On the outside (towards the bark) it will form phloem tissue (living, actively “pumping” cells, and support cells) that translocate resources (transport both ways, up and down)
only the newly formed xylem will transport upwards over the graft union. The old cut xylem no longer does anything.
disclaimer and sources
In the above text you should read secondary xylem everywhere if written xylem. I left out the secondary everywhere to avoid confusion. Since the difference between primary and secondary xylem is not relevant here. (for completeness sake. the primary xylem is formed by the apical meristem (the growing tip of a shoot) while the secondary xylem is formed from the lateral meristem’s cells (in the vascular cambium)
see for example the following sources for more info/explanation. (if oversimplified some things)
That would be hard not to accomplish while joining cambiums.
i disagree, it will be virtually impossible to accomplish.
Once xylem (long dead straws inside the woody part of the shoot) are cut. you cannot reconnect them. And connecting them “sideways” has little use.
You connect the cambium. And the cambium grows cells that differentiate into new xylem the old xylem will stay unconnected!
This is a great topic the answer is green. Went through a terribly frustrating process with this years ago when grafting some 6 or more Japanese plums to western sand cherries. The problem was the layer was so thin on 1 or more of the plums I was concerned I would fail at it as there was no room for error. The best way to approach this is by literally breaking every layer and what they do down in detail. This website breaks the layers and their purpose What are the Layers of a Tree Trunk? | Complete Tree Care
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Posted on [July 12, 2019]
Parts of a Tree Trunk
As mentioned, tree trunks have 5 separate layers to them. They are the outer bark, inner bark (phloem), the cambium cell layer, sapwood, and heartwood. Each layer has their very own purpose, but overall, the trunk’s primary job is to protect and support the tree. Look below to review each layer and what they do.
Like a shield, a trunk’s outer bark is there to protect the tree from its outside surroundings, including inclement weather, wildlife, pests, and more. It also controls moisture, by both preventing excess moisture in the rain and snow, and retaining sufficient moisture levels during dry seasons. It also provides insulation in cold weather and protects against sunburn in the summer.
The phloem, or inner layer of bark, is where food and nutrients are passed through the tree. This layer has a very important job, but a very short lifespan. It eventually dies, turns to cork, and becomes part of the outer layer of bark!
The cambium cell layer is interesting because it is the part of the trunk that grows. Each year, this layer produces more bark and wood as a reaction to the hormones being passed down from the leaves along the food pipeline. These hormones are called auxins, and they are very important because they stimulate new cell growth!
Sapwood is new wood, and serves an important role as the tree’s water pipeline, delivering water to the entire tree. And as new sapwood is created, the inner cells lose their vigor and turn to heartwood.
Heartwood is the most inner part of the trunk. It plays an important role in balance, stability, and security for a tree. Technically, heartwood is dead, but it does not atrophy or decay (unless the outer layers are jeopardized). It is made up of a hollow, needle-like cellulose fibers that are joined together by a glue-like chemical called lignin.
The pith is made of soft, spongy parenchyma cells, which serve the purpose of storing and transporting nutrients throughout the tree. The medullary rays are cellular structures found in only some tree species., and appear perpendicular to the growth rings. The growth rings, also known as Dendrochronology, appear one at a time, once per year, darker in the fall and lighter in the spring and summer."
When @fruitnut showed me how to tbud he expressed the importance of leaving the inner wood behind and taking only the outer layers skin wiyh the tissue. We don’t want heartwood or sapwood.
We only want the outside skin of the tree when making tbuds
There was a time when I first began grafting when I felt it was important to peel the vascular cambium from the sapwood and let me say grafts failed 100% of the time. The sapwood needs to go with the cambium in the case of tbudding.
That makes sense, but how do you know? The cells are not all dead- are you sure the xylem parenchyma don’t play a part in scion survival?
To what specifically are you referring?
True, there are living cells in the xylem. they are however not dividing meristem cells. And thus no new xylem connections can be formed inside the cut xylem.
It’s hard to prove a negative. I can’t prove xylem parenchyma cells don’t play a role in grafting success. Just like i can’t prove “invisible ray lazers from aliens” aren’t a factor in grafting success. Although that 2e example being ludicrously overexaggerated ofcourse.
I have grafted small pieces of bark directly to other bark (without xylem in between) and that works. There might have been a few xylem cells though. Or that it works without xylem does not prove xylem has no effect. Again hard to “prove” a negative.
can you prove the “positive though”?
That should be a lot easier.
That PDF is awesome. Thanks for linking it 
if i could 
your post with that PDF more than once i would !
I don’t know if everyone noticed, but this topic was a resurrected old thread.
Still, some interesting new comments.
Clark, I think if you’ll examine all the info on this thread. The cambium can not generally be differentiated by the color green. Looking for a green color will actually mislead new grafters.
I’ve had a lot more experience grafting since I started this thread 7 years ago. You definitely don’t want to try to match up your small chip of a bud with anything green on the rootstock.
We always remove the heartwood from the t-bud chip. It generally pops right out. Imo, it makes for more successful takes vs. leaving it in like chip budding. Last fall we t-budded about 150 peach rootstocks (two t-buds per rootstock). I think out of all those, we had one rootstock which both buds failed. We get poorer results with chip budding.
Technically that’s true but normally it’s green. That was the problem with one of those plums. Exposing the cambium there was no green to be seen or anything resembling cambium. Side grafting it to a Hansen cherry is challenging when I couldnt line it up. I’m referencing this experiment I did 8 years ago "What plum scion wood to use on prunus besseyi ?
clarkinks(5b)
January 25, 2014
What is your opinion of the best plum wood to graft on a western sand cherry? The base is about the size of my wrist. I was planning to use a rind grafting technique. What are your thoughts?
clarkinks(5b)
January 26, 2014 at 10:10AM
Wanted to give a quick update as it may be useful to someone in the future. We decided to use Alderman, Toka, Superior, Kahinta, Waneta, and Gracious scion wood to rind graft over the existing sand cherries because they are hybrids. " garden web since has removed many of those old experiments we did which is unfortunate Compatibility grafting? - #23 by clarkinks
i agree that they will never reconnect as a contiguous entity(being dead material), but what tried to say was that it is important that scion xylem and rootstock xylem be in good contact with no air spaces, no debris, in-between. I mean, cambium contact can wait, but sapwood contact cannot, as evidently, if we envision the girdled elm trees posted by OP, the entire canopy(which we can envision as a hypothetical graft) of the elm tree was alive for two years despite no cambium contact, being supported merely by dead sapwood/xylem. Have to add that just because xylem was severed, it does not mean sap can’t flow upwards between scion and rootstock. what matters is that we max out contact between the two with minimal air gaps/debris so the flow is facilitated.
i guess we are a trio in disagreement. Although i partly agree with you both. When we whittle scion wood, especially those with hard sapwood(persimmon, jujube, etc), we tend to make several cuts. And even with one smooth cut, each cut doesn’t necessarily mean it is a planar cut, so the surface will be uneven. Which means there will be air gaps between them. Cutting invoves production of debris, which will result in microscopic blockage obstacles. The act of cutting also involves exposure to air, which result in gumming of sap(think latex), which in turn will be a form of debris that limit contact between scion xylem and rootstock xylem.
i also agree that cambium will differentiate into new xylem, but even though the rootstock xylem is old, it will still be the only source of sap which scion can rely on at the beginning of a graft. New xylem literally has to die first before it becomes functional as sapwood.
This topic seems straightforward but I think the document I posted is to detailed for this discussion I’m breaking it out into a seperate topic. The document contains details not widely available that apply to grafting Grafting and callusing explained in every detail
Thanks Olpea in particular for your posting and comment, pointing out that cambium is not always green! My example that illustrates your point involves my experience with top working sweet cherry trees. I began two years ago. At the time all I had researched indicated to me that the cambium layer is always the green layer below the outer bark. So I followed that hint making sure that my scions were in contact with that layer. The bark was easily separating from that green layer so I was convinced it must be cambium. Of course non of my grafts, wedge grafts, t buds or chip buds worked. After they failed the first year, I began to attribute my failure to various factors such as wrong time, wrong season, wrong temperature, but I could not point to a certain reason. Now after reading back thru this thread and all the videos, particularly your posting of the wedge graft, I understand that on my sweet cherry the cambium layer is below the bark cork which and the green layer between the outer bark and the cork is not cambium.
So I hope other’s just starting out to graft have a chance to read your post!
Thanks again to all contributors in this rejuvenated thread for the information you imparted.
Dennis
Kent, wa
the cambium is always next to the sapwood (xylem). Thus if you can peel of the bark (bark is slipping) where your peeling, that’s where the cambium is.
This is where i disagree.
Sapwood contact will do nothing. There will be no sapflow once the xylem has been severed.
The new cambium contact will form first, then you’ll get new phloem contact a few days later. And a few days after that you’ll get newly formed xylem forming the final “bridge” and making the graft complete.
In your elm tree example. The xylem/sapwood has not been severed. And thus it’s completely different from a graft.
If the xylem is severed it exactly means there is no more sapflow in the xylem. And thus no more sap flow between scion and rootstock.
The first new sapflow that can happen is after the callous tissue’s touch and the first new phloem has formed. After that new xylem will form, and than you have restored all the severed connections.
Before the new phloem has formed after the cambium callous has connected, there is no contact between scion and rootstock. The scion is surviving on stored resources and moisture. That’s why it helps so much to avoid the scion loosing moisture.
Nope. As soon as the cells have differentiated to xylem, it starts fulfilling it’s function. It does not have to die first.
Think of it this way. If you where to do a graft. Where you replaced a little piece of the sapwood with something to block it. (bit of wax?) but have the bark (including cambium) bridge that blockade. Would you expect the graft to survive long term?
And what about if you did a graft without the bark/cambium. only connecting xylem/sapwood and than wrapping it. Do you expect that graft to survive for more than a few months? (might survive a little while on stored moisture and resources)
If your correct. that graft without cambium would survive as long as there is a branch below the graft “feeding” the roots to keep the roots alive.
If I’m correct, the graft will die within months.
while we both agree that xylem is dead material, the difference between our discussion is that you think xylem won’t channel sap upwards if totally severed and then re-attached. I actually think it will continue to do so and that it is in fact crucial to survival of the scionwood more than anything else.
it may still survive since sap literally upwells, and will flow upwards (but only peripheraly, since the sapwood has been blocked) into scion cambium since there will be contact, but i actually think it won’t be as vigorous in growth compared to a scion that has a high surface area of sapwood snugly in contact with each other.
should have mentioned this earlier, but have already done this a long time ago, and proven it to work.
as long as you seal the wound/junction air-tight(wrapping with something that is durable/does not decompose), it will work. I even have soft findings indicating the girdled scion to be more fruitful on its frist or second years compared to grafts where cambiums are well-approimated , since the scions are hogging all their products of photosynthesis and won’t have to share with the roots. If you try it, you;d also notice that the scion will get thicker in girth compared to the rootstock, especially if you decapitate the rootstock, graft the girdled scion, and not allow the rootstock to activate nodes to leaf out from below the graft
lastly, xylem attains full functional status when it has fully differentiated(fully dead, that is), and not fully funtional while it is still alive.
but it, too, just like most biological conduits, will attain a peak, and then regress. It is atherosclerosis in humans, and full lignification into heartwood in plants. The reason why trees live long is that they continue to produce sapwood, with humans we are only assigned one aorta, and this aorta and all its branches get gunky at some point
