12 



THE COMPOSITION OF LIVING ORGANISMS. 



and therefore the first step to be taken in the study of biology is 

 to examine how the matter of a living organism is arranged. 

 This may be done conveniently by a preliminary study of the 



FIG. 1. (After Sachs.) Longitudinal section through the growing apex of a young pine- 

 shoot. The dotted portion represents the protoplasm, the narrow lines being the par- 

 tition walls composed of cellulose (C 6 Hi O 5 ). (Highly magnified.) 



general composition of living organisms, leaving out of consider- 

 ation, for the present, the lowest and simplest forms, and con- 

 sidering only those more highly organized plants and animals 

 which are in a greater or less degree familiar to every one. 



r, 



w 



FIG. a. Cross-section through part of the young leaf of a fern (Pteris aqnilina), showing 

 thick-walled cells; mostof the walls are double. The granular substance is protoplasm. 

 Most of the cells contain a large central cavity (vacuole) filled with sap, the protoplasm 

 having been reduced to a thin layer inside the partitions. Nuclei are shown hi some of 

 the cells, and lifeless grains of starch in others: n, nuclei; s, starch; r, vacuole; n\ double 

 partition-wall. ( x 500. ) 



Organisms composed of Organs. Functions. It is characteristic 

 of any living body for example, a rabbit or a geranium that it 

 is composed of unlike parts, whose structure enables them to 



