HOW PLANTS AND ANIMALS GROW. 509 



Plant cells are not independent units as assumed in the cell 

 theory of organic structure, as recent investigations, with im- 

 proved microscopes and more exact methods of research, have 

 shown that the protoplasm of adjacent cells is connected by slen- 

 der threads which pass through minute openings in the cell walls, 

 and this has been observed in so many cases that the continuity 

 of their protoplasm is believed to be the rule in the structure of 

 plants. The various tissues and cells of the higher plants have, 

 therefore, a common bond of union in the connecting threads of 

 protoplasm which determine their harmonious action. 



The higher powers of the microscope likewise show that the 

 protoplasm of plants is not homogeneous, but contains numerous 

 granules which repeat themselves indefinitely by a process of 

 self-division, each granule having a genetic relation to pre-exist- 

 ing granules of the same kind. Besides the granules, each proto- 

 plasmic cell has a nucleus which in the same manner is formed 

 by the self -division of a pre-existing nucleus. The granules, and 

 especially the nucleus, may prove to be important factors in the 

 perpetuation of ancestral characters, and consequently more inti- 

 mately involved than other elements in the grand mystery of life. 



The chlorophyll granules which constitute the green coloring 

 matter of plants were supposed to be formed from the proto- 

 plasm in which they appear ; but they are now known to arise 

 from the pre-existing self -propagating granules of protoplasm. 



The conception of ascending steps of constructive metabolism 

 resulting in the formation of protoplasm and the storing of 

 energy, with correlated descending steps, by which protoplasm is 

 transformed into less complex compounds (destructive metabo- 

 lism), with a liberation of energy, serves as a key to the complex 

 processes of nutrition which enables us to trace their conformity 

 to general laws that are readily recognized, and clears up the ob- 

 scurity arising from the multiplicity of details which from other 

 points of view could not be brought into consistent relations. 



In the light of these principles the relations of protoplasm to 

 the leading features of vegetable nutrition may be traced in brief 

 outlines, as a prelude to the more highly differentiated processes 

 of animal nutrition. The latest discoveries in physiology all tend 

 to verify the conclusion that the simple chemical elements and 

 binary compounds, which constitute the food of plants, are built 

 up by successive steps of gradually increasing complexity into 

 protoplasm, or living substance, the ultimate product of construc- 

 tive metabolism, and that the energy expended in the work per- 

 formed is stored in the form of potential energy as an essential 

 element or condition of its constitution. From the instability of 

 the exceedingly complex molecules of protoplasm, destructive 

 metabolism immediately follows, and the proximate constituents 



