One of the very first things we physicians do upon seeing a patient is to investigate his circulation. If the pressure is too low or too high, for any reason, we immediately take measures to correct it, because we know that disastrous results will quickly follow if that is not looked after. Plant circulation, or sap flow, is no less important. Mr. Riehl, Mr. Jones and Dr. Morris made great strides when they advanced the ideas of covering the wound and the scion completely to prevent evaporation, thereby also controlling the sap pressure. With the exception of shading, pruning and defoliating, this is about the only method we have of preventing evaporation. Defoliation, of course, interferes with the tree's power of growth. Controlling the humidity is probably not practical on a large scale.
A proper and careful cutting of the tree beforehand is important. It appears that to cut the top completely out while the tree is dormant, so disrupts the routine circulation that the few lower branches which are left intact, are well taken care of and, it seems to me, that this, together with the stimulation of WOUND REPAIR by cutting and allowing time enough for the cells to get into action, was the prime reason for the 100% success in the three Hales and the cause of the 100% failure in the other Hales tree.
Other methods of controlling the circulation are of course drainage, irrigation, mulching, location of the orchard, placing of condensers of moisture, such as stones and other hard substances beneath the trees, and many other contrivances which are in use, and which I shall not discuss.
With reference to stimulation of cellular activity we are considerably concerned. In medicine I have found the subject of wound repair and immunity most interesting, the two subjects seeming to be more or less related. Some animals will repair wounds and immunize readily, while others will not. In a general way young healthy animals and human beings immunize most readily, while older ones frequently fail almost entirely. Interestingly enough plants seem to be strangely similar in this respect, and the thing that stimulates cellular activity for defensive purposes (immunity) apparently stimulates growth and wound repair. The thing that stimulates most actively for a special purpose is the thing itself, the best stimulant for wound repair being the simple injury. To illustrate briefly: In my work last summer I came in contact with two enemies, yellow jackets and copperheads. The copperhead stimulated me to carry a club in defense, while for the yellow jacket the club was of little value and I rather preferred carbon bisulphide. Had I ignored my senses and allowed nature full sway, as a tree does, the snake would have injected his venom and the yellow jacket his toxin, and my cells would have accepted their only alternative and proceeded at once to build up a specific defense, after which they would have been in better shape for development, providing the poison would not have been so great as to prove fatal. Injury to a tree certainly does stimulate wound repair, defense and growth. It is well known that trees with many transplantings, root injuries, transplant much more readily, and the nurserymen use this method of stimulation as a routine procedure. I learn in Florida that in order to transplant a good size palmetto, they are in the habit of digging down on one side and cutting the roots the year before removal. It will then transplant more readily. Pruning has the same cell stimulating effect if done at a time that will retain the stored nutrition. An attack of disease just as surely stimulates cellular activity and growth but it is too frequently followed by disaster.
We have all heard of driving rusty nails into trees (thinking the iron produced the beneficial results), cutting a slit in the bark of the limbs and trunk for "bark bound" so called, etc., all of which have stimulating effects with more or less permanent injury to the tree. Who knows but what the sap sucker, with his ability to dig into the bark and extract a piece of cambium, was not sent to us to aid in preserving our trees by stimulating new growth?
In my work last summer trees that were subjected to slight injury before hand apparently accepted a larger proportion of grafts. I will briefly cite two specific illustrations. A little butternut tree located near the house was the object of my efforts for over two years. During my illness I frequently went out and pruned a few branches or put on a few buds. Something would happen to me and possibly I would not see it again for months, and in the meantime the buds would be strangled or knocked off. Another little hickory tree stood in the roadway. Harrows, plows, wagons and even logs were dragged over it. Grafts on both these trees caught rather readily last spring. In fact two black walnut grafts on this little butternut were two of the very few that I got to grow at all last year. My walnut grafting was almost a total failure. I have this to say, however, that I had no dormant walnut scions, my scions all being cut in May or June.
Mr. Jones, by marking the site of his patch bud several days in advance, admirably carries out this idea by locally stimulating the cambium cells. Dr. Morris's scheme of using white wax, besides regulating sap pressure, allows the actinic rays of the sun to stimulate cellular activity. Cutting the top out of the tree, which disrupts the normal circulation and throws it into the few lower limbs, besides stimulating the cells into activity, has apparently in a large measure accounted for the slight success that I have had. Other methods such as injecting some substance under the bark, applying antiseptics, or some stimulating chemical in a similar way, as "Scarlet Red" is used in skin grafting to increase epithelial growth, may aid materially. Certain chemicals applied to the tree and leaves, as used in sprays, seems sometimes to stimulate growth in a way that can hardly always be accounted for by the checking of the disease for which it was placed.
Much more could be written on cellular stimulation but enough has been said to encourage others to make observation in this connection, for it is highly probable that the lack of proper stimulation of the cambium accounts for more failures in top working trees than we are aware of.
3RD CONTROL OF PLANT CELL NUTRITION
With this topic we are probably less concerned in its relation to grafting than when the growing and bearing stages come. However, certain nutritional disturbances appear early and the more vigorously the stock is growing beforehand the better progress, of course, the grafts will make when they are started. Whether or not they will start more readily have I been unable to ascertain, but I have a bunch of little fellows with a growth of only an inch or so, and so puny that I cannot account for it in any other way than a lack of proper nutrition. Many of these little trees, used as stock, are very old in comparison with their size and they will probably be dwarfs all their lives. It is a question whether many such trees should be grafted at all. Further observations will have to be made to decide that point. Perhaps proper preparation for a year or two would be beneficial.