14 



THE COMPOSITION OF LIVING ORGANISMS. 



ing widely in structure and function. Outside are the skin, the 

 hairs, the nails ; inside are bones, muscles, tendons, ligaments, 

 blood-vessels, and nerves. The leaf of a plant is an organ consist- 

 ing of a woody framework (the veins) which supports a green pulp, 

 the whole being covered on the outside by a delicate transparent 

 skin. In like manner every organ of a higher plant or animal 

 may be resolved into different parts, and these are known as 



FIG. 4. Cross-section through dead wood-like cells from the underground stem of a fern 

 (Pteris aquilina). The walls are uucoiiiinouly thick and the protoplasm has disappeared, 

 (x 450.) 



tissues. The tissues of fully formed organs are often very differ- 

 ent from one another, as in the cases just mentioned ; that is, they 

 are well differentiated; but frequently in adult organs, and 

 always in those- which are sufficiently young, the tissues shade 

 gradually into one another, so that no dHhnte line can be drawn 

 between them. In such cases they arc 1 said to be less differenti- 

 ated. For example, in the full-grown leaf of a plant the woody 



