2 NATURAL HISTORY. 



will kill all animals and plants, while the amount of cold which a few- 

 are capable of withstanding in a dry state has not definitely been 

 ascertained. 



The line between living and non-living matter is well marked ; not 

 so that between the two great groups of living bodies, — animals and 

 plants. With the higher forms no difficulty is experienced in making a 

 separation, but when we study some of the more minute and simpler 

 species, the characters on which we before relied lose much of their value. 

 A sensitive-plant or a sundew seems to have more sensibility than some 

 forms which are undoubted animals ; many of the lower plants move 

 about freely, while the coral is as firmly fixed as any tree. And so on 

 through the list : each character has its exceptions. Plants as a rule 

 derive their nourishment from the non-living world ; but the group of 

 fungi (mushrooms, moulds, etc.) are like animals, in that they must have 

 living bodies or the products of life for food. So far as is known, there 

 is no undoubted animal which can convert non-living matter into proto- 

 plasm. With this brief statement we may dismiss the subject, for in 

 almost every case which will occur to any but the special student, the 

 ordinary distinctions between animals and plants will prove valid. 



The protoplasm of which an animal body is composed is not in one 

 solid mass. Just as a brick house is built of numbers of separate and 

 distinct bricks, so one of the higher animals (or plants) is formed by the 

 union of individual and distinct bits of protoplasm. Each of these 

 bricks of the animal structure is known as a cell. Here, however, the 

 simile ends, for while a brick is of the same constitution throughout, a 

 cell is a complex body, and under the microscope we can distinguish 

 various structures which chemical tests show to vary in composition. 



First, the bulk of the cell is made up of proto- 

 plasm, while inside of this a smaller mass is distin- 

 guishable, usually nearly spherical in - shape, — the 

 nucleus. In the older works, smaller particles — 

 nucleoli — were described inside the nucleus, but 

 now it is known that these minute spots were 

 optical sections of nuclear filaments. The accom- 

 panying diagram illustrates the general features of 

 Fl s G howi^ D thfcTii-w f an (J)! a cel1 ' Som etimes a limiting membrane, cell-wall, 

 ^.S^XSkHSKS surrounds the whole cell, but this may occasionally 



the nuclear filaments (/). be wanting> 



In still another respect our simile fails : a single brick does not make 

 a house, while an animal (or a plant) may consist of a single cell which 

 exercises in itself all the functions which are, in the higher animals or 



