LESSONS IN BOTANY. 245 



The air with its changing temperatures, the water with its 

 multiform impurities, the soil with its different qualities, the ex- 

 istence of other plants and animals, all constitute an endless 

 variety of conditions under which plants exist; and which plants 

 have been made to fit, either by their creation in special forms, or 

 in a form adaptable to all the different conditions that might 

 arise. And perhaps here lies the answer to our query the form, 

 color, fragrance and other peculiarities of plants are, or may be, 

 determined by their surroundings. 



The lowest groups of plants, those which are supposed to have 

 appeared first on the earth, are water plants, or are active only 

 when surrounded by an abundance of moisture. The air and 

 water contain all the nutriment necessary to these, so that there 

 is no call for special organs of nutrition, each cell performing all 

 the functions of a plant, and as there is no division of labor the 

 whole body is an organ of nutrition, and of reproduction as 

 well. The ability to survive long periods of dessication, as in 

 the case of protococcus and many others, implies some modifi- 

 cation, some improvement over the organism of strictly aquatic 

 plants. 



As the rocks were broken down and pulverized into soil, plant 

 life began to appear on the land. But water, one of the essen- 

 tials of plant life, was not now present on all sides; the supply 

 was in a sense limited, coming mainly from the soil below. It 

 was not necessary for the upper surface to be fitted to absorb 

 moisture, but it was necessary to so cover it as to prevent the 

 evaporation of moisture, and hence the plant is differentiated 

 into parts, consisting of a body from whose under surface roots 

 are developed to take up moisture and other nourishment, and 

 on the upper surface an epidermis is formed which protects the 

 body from excessive evaporation. 



Another interesting modification arose when plants, as spiro- 

 gyra, in addition to reproduction by simple division became able 

 to form resting spores, which enabled the species to survive ex- 

 treme cold or drought, thus greatly extending the range of plant 

 life. These examples illustrate the first steps in the differentia- 



