122 AGRICULTURE OF MAINE. 



white of an egg than anything else we meet with in every day 

 hfe. Except in the youngest cells this cytoplasm forms a thin 

 layer just within and in close contact with the cell wall, and 

 in it the different processes of food manufacture, transforma- 

 tion and nutrition go on. Imbedded in it are various small 

 bodies called plastids. Some of these are known to perform 

 certain definite work in the cell. Two classes of these which 

 are very much alike in many ways are of especial interest to 

 us. Tlie green ones or chloroplasts which are so abundant 

 in the cells of the leaves as to give them their green color 

 are the bodies which are concerned in the manufacture of 

 starch. Other colorless ones in cells in other parts of the 

 tree or in portions of the trunk and roots are the agents 

 which store up the starch in those cells for a future food 

 supply. The cavity within the cell is filled with the cell sap 

 which consists largely of water with other substances in solu- 

 tion. Somewhere within the cell, sometimes suspended in the 

 cavity or often at one side, but always connected with the layer 

 of c^-toplasm within the cell wall, is a very definite body called 

 the nucleus. While this in many respects is by far the most 

 important of all the parts that go to make up the cell, in that 

 it is considered by many to be the controlling factor in the 

 various vital activities of the cell and plays a very important 

 role in the transmission of hereditary characters, it is not neces- 

 sary for our present purpose to discuss it farther. 



The cells which go to make up the root hairs are long and 

 slender and are very thin walled. Those which go to make up 

 the tissues of the root, trunk and branches are variously modi- 

 fied. Many of the wood cells are long and slender with pointed 

 ends which overlap each other, breaking joints, anrl thus giving 

 strength. In other cases the end walls of larger cells in the 

 woody tissues disappear along with their living contents. To all 

 intents and purposes much of the interior of the trunk is dead 

 tissue, but it is by no means functionless. The disappearance 

 of the end walls of the large cells leads to the formation of 

 large tubes or ducts, running up and down the stem, and these 

 serve a most useful purpose in helping to provide a passage for 

 the food materials in solution from the roots below to the 

 leaves above. They may be roughly compared to a system of 

 pipe lines. Some of them are curiously pitted while otliers are 



