12 



Trees, Stars, and Birds 



FIG. 7. Cells from a moss leaf. 



usually watery sap and often starch or other food material 

 stored in a plant cell, but only the protoplasm is alive. 



Although most cells are too 

 small to be seen with the 

 naked eye, the cells in such 

 material as the thin leaf of a 

 moss or the skin from the 

 bulb of an onion may easily 

 be seen with a compound 

 microscope. A tree, there- 

 fore, though it appears to 

 be but a single thing, is in 

 reality made of many small 

 parts. A leaf is like a great 

 structure built up of many 

 hundreds of little boxes, and 

 all the other living parts of a tree are, in the same way, 

 built of many parts. 



How the wood is formed in a tree. The cells in the 

 tip of a growing shoot or root of a tree are all alike, but 

 as they grow older great changes take place in many 

 of them. Some of them grow wider and at the same time 

 become greatly elongated. Then the cell walls are 

 thickened by woody material being laid down on the 

 inside of them. Finally, the end walls are absorbed, 

 so that each cell is connected with the one above and 

 below to form a long tube. The living matter in the 

 cells then dies, leaving long, empty vessels, sometimes 

 several inches or even several feet in length, made. of 

 cells joined end to end within the plant. These vessels 

 are called trachece (singular, trachea). Through them 



