16 THE COMPOSITION OF LIVING ORGANISMS. 



"What has been said thus far applies only to higher plants and 

 animals. But it is an interesting and suggestive fact that there 

 are also innumerable isolated cells, both vegetal and animal, 

 which are able to carry on an independent existence as one-celled 

 plants or animals. Physiologically these must certainly be re- 

 garded as individuals ; but it is no less certain that 

 they are equivalent, morphologically, to the constituent 

 cells of ordinary many-celled organisms. It will appear 

 hereafter that the study of such unicellular organisms 

 forms the logical groundwork of all biological science. 



Since organisms may be resolved successively into 

 organs, tissues, and cells, it is evident that cells must 

 contain living matter. And a cell may be defined as a 

 small mass of living matter either living apart or form- 

 ing one of the ultimate units of an organism. 



Lifeless Matter in the Living Tissues. In the tissues 

 the living cells are not, as a rule, in immediate contact 

 one with another, but are more or less completely sepa- 

 rated 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 cells separated by very 

 narrow partitions. Each cell consists of a mass of ~ 



granular, viscid, living substance known as protoplasm, 

 part of which is condensed into a 

 more solid, large, and rounded body, 

 the nucleus. 



In such a group of cells no tis- 

 sues can be distinguished ; or, rather, Sl^iv^i 

 the whole -mass consists of a single 

 tissue (meristem), which is almost en- 

 tirely Composed of living matter (pro- FlG . 5. (Aft er Ranvier.)-Muscle-ceils; 

 tonlasm). In older tissues the parti- A > from the ^testine of a dog, in 



. ' . . cross-section; B, single isolated cell. 



tlOllS Olteil increase 111 thickness, as from the intestine of a rabbit, viewed 



shown in Fig. i>. In every case the fromtheside - <* m > 

 partitions are composed of lifeless matter which has been manu- 

 factured and deposited ly the living protoplasm constituting 

 the 1<>diex of lit, CC//N. In still <>k)er parts of the plant certain 

 of the lifeless walls may become extremely thick, the protoplasm 

 entirely disappears, and the whole tissue (wood) consists of life- 



