THE CELL 19 



(d) Quite commonly, in plants, adjacent protoplasts 

 are Joined together by strands of cytoplasm passing 

 through minute pores in the cell-wall. 



24. Peculiar Properties of Protoplasm. — More is known 

 of the structure of protoplasm than is indicated above, 

 but a more detailed treatment is reserved until Chapter X. 

 Quite as important as the structure of protoplasm are the 

 physiological or functional characteristics, or properties, 

 that distinguish it from every other known substance. 

 The most significant and wonderful of these is its ability 

 to reproduce itself. By the vital activities of animals 

 and plants, their living substance undergoes a continual 

 destruction, which is accompanied by continual construc- 

 tion. Parts which are destroyed are constantly replaced, 

 and new protoplasm is continually being formed. No 

 other known substance can do this. If a crystal, for ex- 

 ample, of salt, is suspended in a saturated solution of 

 salt in water, some of the salt particles in solution will 

 attach themselves to the crystal in a regular manner, so 

 as to enlarge it, while preserving its characteristic shape. 

 But here, as is readily recognized, the crystal itself is 

 entirely inactive. It does not change another kind of 

 matter into salt, but merely serves as a center of deposit 

 for more salt. Protoplasm, on the other hand, entirely 

 alters the nature of the substances which enter into it, 

 and recombines them into a substance like itself, with 

 entirely new properties — in fact converts the non-living 

 into the living. 



25. Secretions. — In the course of its continual de- 

 struction and reconstruction, protoplasm gives off or 

 secretes other substances, unlike itself and unlike the 

 material of which it was formed; these are called 



