TISSUES AND CELLS DEFINED. 15 



framework, the green cells, and the skin exist as three plainly 

 different tissues. But in younger leaves these same tissues are 

 less different, and in very young leaves, still in the bud, there are 

 no visible differences and the whole organ is very nearly homo- 

 geneous. In this case the tissues are imdifferentiated, though 

 potentially capable of differentiation. In the same way, the 

 tissues of the embryonic human hand are imperfectly differen- 

 tiated, and at a very early stage are undifferentiated. 



Tissues composed of Cells. Finally, microscopical examination 

 shows every tissue to be composed of minute parts known as 

 cells, which are nearly or quite similar to one another throughout 

 the whole tissue, and form the ultimate units out of which the 

 tissues and organs, and hence the whole organism, are built, 

 somewhat as a house is built of bricks or pieces of wood. 



It will be shown in Chapter IV. that the ultimate units or 

 ceils possess everywhere the same fundamental structure; but 

 they differ immensely in form, size, and mode of action, not only 

 in different animals and plants, but even in different parts of the 

 same individual. As a rule, the cells of any given tissue are 

 closely similar one to another and are devoted to the same func- 

 tion, but differ from those of other tissues in form, size, arrange- 

 ment, and especially in function. Indeed, the differences be- 

 tween tissues are merely the outcome of the differences between 

 the cells composing them. The skin of the hand differs in ap- 

 pearance and uses from the muscle which it covers, because 

 skin-cells differ from muscle-cells in form, size, color, function, 

 etc. Hence a tissue may be denned as a group of similar cells 

 having a similar function.* As a rule, each organ consists of 

 several such groups of cells or tissues, but, as stated above, young 

 organs are nearly or quite homogeneous ; that is, all of the cells 

 are nearly or quite alike. It is only when the organ grows 

 older that the cells become different and arrange themselves in 

 different groups, a process known as the differentiation of 

 the tissues. In the case of some organs for instance the leaf 

 of a moss the cells always remain nearly alike, somewhat as in 

 the embryonic condition, and the whole organ consists of a 

 single tissue. 



* Tissues frequently contain matters deposited between cells ; but these 

 have been directly derived from the cells, and vary as the cells vary. 



