40 STATE POMOLOGICAL SOCIETY. 



of gas or air. The gas content of the wood on some trees being 

 as high as 20 to 25 per cent of its volume. The filrri of pro- 

 toplasm, like the cell wall, allows water to pass through it but 

 it can regulate in a great measure the passage of the soluble cell 

 contents. If, however, this living matter is killed, for example 

 by frost, the cell sap readily passes through it and escapes. 

 This makes the tissues soft and flabby as is the case with frozen 

 leaves of tender plants. 



In a tree like the apple or pear, the living, active cells are 

 largely confined to the outer layers of the wood and inner layers 

 of the bark. Most of those in the interior contain no living 

 protoplasm and are functionless except as they assist in support- 

 ing the trunk and in the transference of liquids. The region 

 of growth in the tree, as we all know, is at the junction of the 

 bark and wood. This is known as the cambium zone. During' 

 the growing season the cells of the cambium layer grow rapidly 

 and are actively dividing, forming wood on one side and bark on 

 the other. The wood formed in any one season from this divis- 

 ion produces the well marked annual ring which we all recog- 

 nize. If anything happens to destroy the living cells in the 

 cambium zone, growth stops there, the bark dries down or 

 sloughs off and we have an exposed surface or wound inviting 

 the entrance of wood decaying fungi into the tissues below. 



Before leaving the question of structure we should point out 

 those elements which are concerned in the transference of food 

 materials and foods in the tree, for this throws light upon certain 

 phenomena observed with reference to frost injured trees 

 ]\Iineral food substances dissolved in water are absorbed by the 

 fine hairs near the ends of the minute rootlets, then carried up 

 through the roots to the stem where they pass upward through 

 the vessels of the wood inside the cambium zone to the leaves. 

 Here they mingle with the gases taken up from the air and are 

 converted into starch and other food materials by the protoplasm 

 of the leaf cells, acted upon by sunlight. Then these manufac- 

 tured food materials are sent back down to furnish nourishment 

 to the growing parts of the trees. But instead of going down 

 through the wood they pass down through certain vessels in the 

 inner part of what we commonly call the bark, next to the cam- 

 bium. The downward course of the manufactured food materials 

 is easily demonstrated, for every orchardist is familiar with the 



