120 LECTURES ON 



particles, (atoms,) in such a manner as not to alter their essential 

 characters. 



The forces of cohesion, gravitation, heat, light, electricity, and 

 magnetism, are physical forces. A thousand fragments of iron, for 

 example, maybe made to cohere together or gravitate to the earth, may 

 be changed in temperature, illuminated, electrified or magnetized, with- 

 out any permanent change in that assemblage of properties which 

 constitutes this metal. 



Chemistry is the science of chemical force or affinity, which causes 

 two or more bodies to unite with the production of a compound pos- 

 sessing essentially new characters. Thus a hard lump of quicklime 

 when brought in contact with water greedily absorbs it, with the pro- 

 duction of great heat, and falls to powder. In slacking, it has combined 

 chemically with water. 



Physiology is the science of the processes of life, which require, in 

 addition to the chemical and most of the physical forces, the co-opera- 

 tion and superintendence of the vital principle. 



The first inquiries in the natural science of agriculture are: What 

 is the plant? Out of what materials, and under what conditions is it 

 formed ? 



The plant is the result of an organism, the germ, which under 

 certain influences begins an independent life, and grows by con- 

 structively adding to itself or assimilating surrounding matter. 



The simplest plant is a single cell, a microscopic vesicle of globular 

 shape, which, after expanding to a certain size, usually produces 

 another similar cell division either by lateral growth or by its own. 



In the chemist's laboratory it is constantly happening that, in the 

 clearest solutions of salts, like the sulphates of soda and magnesia, 

 a flocculent mould, sometimes red, sometimes green, most often 

 white, is formed, which, under the microscope, is seen to be a vege- 

 tation consisting of single cells. The yeast plant (fig. 1) is nothing more 

 Fi §' L than a collection of such cells now 



existing singly, now connected in 

 one line or variously branched. 



The cell is the type of all vege- 

 tation. The most complex plant, a 

 stalk of cane or an oak, is nothing 

 more than an aggregation of myriads 

 of such cells, very variously modi- 

 fied indeed in shape and function, 

 but still all referable to this simple typical form. 



In the same manner that the yeast plant enlarges by budding or 

 splitting into new cells, so do all other plants increase in mass; and 

 thus growth is simply the formation of new cells. 



So far as the studies of the vegetable physiologist enable us to 

 judge, all vegetable cells consist, at least in the early stages of their 

 existence, of an external, thin, but continuous (imperforate) membrane, 

 the celt-wall, consisting of a substance called cellulose, and an interior 

 lining membrane of slimy or half liquid character, variously called 

 the protoplasm, the formative layer, or the primodlal utricle, (fig. 2.) 



