44 BUILDING MATERIAL OF LIVING THINGS 



How Cells were first found. — We have already found that we 

 can distinguish life through its activities. A living organism is 

 sensitive to stimuli, it moves, it grows by assimilating or changing 

 over substances into the same chemical composition as itself, 

 and it reproduces its kind. Is there any other way in which we 



can distinguish living from Ufe- 

 less matter? 



A little over two hundred 

 years ago, a Dutchman, Anton 

 van Leeuwenhoek (la Ven-hook), 

 made a collection of crude 

 magnifying instruments that 

 were the beginnings of our 

 modern microscope. With these 

 instruments he was able to see 

 tiny organisms swimming in 

 drops of pond water, and it is 

 even thought that he first saw 

 living bacteria. From this be- 

 ginning a very complete knowl- 

 edge has been gained concerning 

 the building material of living 

 A compound microscope is used to Organisms. An English doctor, 



magiiify small objects from about 40_to -^^^^^^ Hooke, examined COrk, 

 500 tunes. The upright tube contams ' ' 



lenses. The object is placed on the stage which is the bark of an Oak 



thr/the"oS. too'ldnf ?own "Sto* tree, and found it was made up 

 the lenses, one should see the object of tiny compartments, like little 



magnified in a clear light. ^^^^^^ ^j^-^j^ ^ie called Cells, a 



term which is now universally used for the unit of structure 

 in living things. 



Protoplasm. — This name cell is not quite descriptive. Hooke 

 saw the dead walls around the spaces that during the life of 

 the plant contained living matter. But it was not until more 

 recent times that biologists found that the content of the cell 

 is the important living substance. This living material has been 

 named pro'toplasm (Gr. protos, first; plasma, formative material). 

 While we rarely see it or feel it, nevertheless observation has shown 

 it to be always present where there is life. It is a sticky, semi-fluid 



