BOTANY. [CHAP. i. 



feeling that the life of a plant must be of a very different 

 nature to that of an animal, say a human being. Such 

 an idea is, from the present standpoint of knowledge, a 

 mistake ; in the broader sense there is but one kind of 

 life, and although a perfect definition of life is at present 

 impossible, yet the important characters which separate 

 it from every other known force are clearly understood 

 and fall under the two headings, growth and reproduction. 

 Growth means increase in size, along with replacement 

 of worn out substance composing the individual, brought 

 about in a special manner due to a combination of 

 chemical and physical processes as follows. It is well 

 known that both plants and animals, when quite young, 

 are very small in bulk compared to the size assumed 

 when full grown, and it is further generally known that 

 growth or increase in size depends on a regular and 

 continuous supply of food. The peculiarity of all forms 

 of life is that the food when first taken into the body 

 differs in chemical composition from the living body, 

 and the latter possesses the property of chemically break- 

 ing up its food, retaining those portions that it requires 

 for the building up of its own body, getting rid of the 

 surplus useless matter. As illustrations of the above, a 

 human being can convert a dinner of bread and beef 

 into human flesh ; in the same way a plant that feeds on 

 carbonic dioxide and water containing certain substances 

 in solution can convert these substances into what may 

 be termed plant flesh. The food taken as a matter of 

 course must contain all the chemical elements required 

 for the formation of animal or plant substances respec- 

 tively, but combined in different proportions and often 

 mixed with other elements not required ; and the pecu- 



