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673 



Principles of Tree Growth 



IN contrast with conditions in animal phybiology, the 

 life processes of plants, and particularly of trees, are 

 understood only in a general way. While the student 

 of animal nutrition can tell with rather close approxima- 

 tion what becomes of the foods the animal consumes, can 

 measure the energy supplied or expended, can follow the 

 building- up and breaking down of various products, the 

 plant physiologist's analagous knowledge is far less de- 

 tailed. Our trees, then, familiar as they are to us, are 

 less comprehended in their ordinary internal economy 

 than are our animals. This lack of definite knowledge 

 is reflected to no small degree in the diversity of opinions 

 on many fundamental questions of practical orchard 

 management. 



Every'tody recognizes the importance of soil as a 

 source of jjlant food and will at once appreciate the im- 

 portant relation of soil to tree. Not so many, however, 

 are aware that plant food is taken into the plant through 

 the roots in liquid form only, consequently a tree may 

 conceivably be starving in the midst of plenty because of 

 a lack of solubility in the abundant food. The mere 

 dumping, then, of a material containing an element of 

 fertilizing value on the soil about a tree does not ensure 

 a hearty immediate feeding for the tree, and in many 

 cases the orchardist, not waiting the time necessary for 

 rendering this food soluble and failing to see any re- 

 sponse at once, concludes that his effort at feeding the 

 tree was unnecessary and therefore wasted. 



Again, a rather common sight is the accumulation of 

 fertilizer or manure near the base of the tree. This is 

 quite the usual thing, this putting the food near by so 

 the tree can apparently get it conveniently. However, 

 the main part of the absorption of raw plant foods is 

 efifected further out from the tree, where the small roots 

 are more abundant. The rootlets that actually absorb the 

 food are formed anew each year from the smaller roots ; 

 the main roots near the tree trunk do not grow these 

 rootlets except on a few straggling branches. This ar- 

 rangement is not always understood because the root sys- 

 tem is almost entirely hidden from ordinary observation. 

 Rut it should be remembered that the roots extend 

 laterally as far from the trunk as the branches and that 

 fertilizer is more profitably applied at some distance 

 from the tree. 



Aside from the soil with its nutrients and solutions, 

 there is, however, another source of food supply that is 

 frequently overlooked, possibly in part because it is so 

 freely abundant that in ordinary orchard conditions it 

 needs little consideration. Chemical analysis shows that 

 a large part of the bulk of any tree is composed of carbon 

 compounds, yet carbon is not known to be taken up by 

 the roots at all. It occurs in the soil, but there it cannot 

 be taken into the roots; it abounds in gaseous combina- 

 tion in the air. From the air then it is taken into the 

 leaves where under the action of sunlight it is broken 

 up; the oxygen which was linked with it is returned to 

 the air while the carbon is retained. The presence of 

 carbon in every smallest piece of the tree shows that 

 it must be moved from the leaves, down intothe wood 

 in some wav since it cannot be taken in otherwise. 



What actuallv occurs is this: the sap taken up by the 

 roots, largely water with its dissolved mineral substances, 

 moves upward through the area known as sapwood. The 

 current extends through all the branchings of the tree 

 until the sap reaches the leaves. Here in the presence 

 of sunlight chlorophvll. or green coloring matter of the 



leaves "digests" this crude sap, combining and breaking 

 down various products, forming several substances of 

 which starches and proteids are perhaps most prominent 

 These accumulate during the day to a greater or less 

 extent in the leaves, but at night when starch formation 

 has ceased the stored supply is moved from the leaves in 

 solution, forming what is known as the "elaborated" 

 sap, flowing downward. 



When a branch is girdled, either by killing of bark, 

 by removal of a ring of bark or by a wire twisted tightly 

 about it, there is a marked change in its behavior. If the 

 injury is not so severe as to prevent healing over there 

 is checking of vegetative or woody growth with a ten- 

 dency toward fruitfulness. The tissue below the girdled 

 part tends to react in the opposite direction ; it is apt to 

 send out suckers or water sprouts. The downward flow 

 of elaborated food material has been interrupted and the 

 two reactions tend to show the relative effects of each. 

 Sometimes the growth of a sucker at a desired spot can 

 be induced by girdling above this point. 



Much of this food material is stored in the tissues of 

 the tree until needed, partly in the twigs and partly in 

 the wood of the larger branches and in the trunk. The 

 medullary rays frequently abound in it. Here it may be 

 drawn upon when needed. No greater mistake can be 

 made than in thinking of the tree in Winter as inert. It 

 is truly dormant, but it is no more dead than is a hibernat- 

 ing woodchuck. All through the Winter it tends to accu- 

 mulate moisture in its tissues until it reaches its maximum 

 content just before the buds break in the Sprmg. h\ 

 some trees the sap accumulates in such quantities as to 

 be actually under considerable pressure, as is shown by 

 the bleeding of these trees when wounded. At such a 

 time sap will travel with considerable speed, as is shown 

 by chemicals introduced as tracers at one point in a tree 

 trunk being recovered at a point a foot above in about ten 

 minutes. 



The tubes through which the sap flows are arranged 

 longtitudinally of the trunk as is indicated by the grain of 

 the wood. Means of direct flow around the tree are not 

 provided and the sap meets some little difficulty in moving 

 to one side or the other. Hence a change in conditions 

 at any given point is not so likely to be felt markedly to 

 one side as it is above or below. 



It has already been indicated that the treatment of the 

 tree must be approached from a somewhat different view- 

 point from that used in handling an animal. The animal 

 is considered as a unit. The same viewpoint perhaps can 

 be held with the tree so far as its soil relations are con- 

 cerned and so far as it is fed through the roots. How- 

 ever, when the orchardist considers the treatment of the 

 top it must be with a more complicated condition in mind. 

 The case of one limb or one part of a tree bearing in 

 alternate years from the rest of that same tree is rather 

 common. This points to a degree of independence in 

 the aerial parts that must be considered in pruning. If 

 all the tree is cut back uniformly the response is uniform, 

 but if part is cut back heavily and the rest undisturbed, 

 or only slightly cut back, the reaction is not uniform 

 This quasi independence must be kept constantly in mind 



In the animal one set of digestive organs supplies the 

 whole body, one heart pum])s the blood, and so on. In 

 the tree one set of roots gathers the crude sap, but there 

 the likeness stops. Each branch, each spur has a strong 



(Continued on page 675) 



