16 THE STRUCTURE OF LIVING THINGS. 



Lifeless Matter in the Living Tissues. In the tissues the liv- 

 ing cells are seldom in contact one with another, but are more or 

 less completely separated by partitions of lifeless matter. This 

 may be seen in a section through some rapidly growing organ 

 like a young shoot (Fig. 1). The whole mass is formed of 

 nearly similar, closely crowded units or cells separated by very 

 narrow partitions. Each cell consists of a mass of granular, 

 viscid, living substance known as protoplasm, and a more solid, 

 rounded body, the nucleus. 



In such a group of cells no tissues can be distinguished ; or, 

 rather, the whole mass consists of a single tissue (meristem), 

 which is almost entirely composed of living matter (protoplasm). 

 In older tissues the partitions often increase in thickness, as 

 shown in Fig. 2. In every case the partitions are composed of 

 lifeless matter which has leen manufactured and deposited by 

 tJie living protoplasm constituting the bodies of the cells. In 

 still older parts of the plant certain of the lifeless walls may 

 become extremely thick, the protoplasm entirely disappears, and 

 the whole tissue (wood) consists of 

 lifeless matter enclosing spaces filled 

 -"^J-^ -'Sy- witli a ' r or water (Figs. 3 and 4). 

 V . ';$p : Among animals analogous cases 



||. are common. The muscles of the 

 small intestine, for instance, (Fig. 

 5,) consist of bundles of elongated 

 V^':^: v cells (jilrefi) each of which is com- 

 posed of living matter surrounded 

 <i|H by a very thin covering (sheath) of 

 ^^> lifeless matter. In cartilage or 

 J >v V ;T ^ -Silt gristle ' wnicn covers the ends of 

 ^ ; ft> . ; ^ man - v 1)011CS ( Fi - )' tlie ova l cells 

 ..->' are very widely separated by the 



^S; - :/. deposition between them of large 



FIG. 8. (Modified from Schenk )-Sec ( * UantitieS f 8 lid ^less "latter 

 tion of bone from the human femur forming what is known as the 



SStflSJlSrss't*^:"** i n Hood ( F ig . 7) ti, e 



ru. Dmgram.tic. flattened or irregular cells (cor- 



flllH , , pmdei) are separated by a lifeless 



1 (plasma) m winch they float. In bone (Fig. s) the cell. 



