ROOT AND SHOOT 99 
inaterial, liquid and gaseous, through its roots and 
leaves. For the present purpose we may take our 
typical plant as consisting of subterranean roots and 
aerial leaves on the one hand, and aerial flowers on 
the other—the roots and leaves concerned especially 
with carrying on the life of the individual, the flowers 
with perpetuating the race. In addition, an aerial 
stem is usually present, on which the leaves and 
flowers are displayed, and through which the food 
materials pass dissolved in water. Of these parts, the 
lower ones (the roots, and sometimes the stems) are 
immersed in the soil, while the upper ones (the leaves 
and the flowers—which are groups of modified leaves— 
and usually the stems) are immersed in the atmo- 
sphere. All the parts have acquired their form and 
fulfil their functions under control of the particular 
medium which surrounds them: it becomes necessary 
to preface any discussion of their characters and 
uses by a brief survey of the characters of these 
envelopes. 
_ While the atmosphere is familiar to us as the 
medium in which we ourselves live and move and have 
our being, and while its chemical and physical proper- 
ties are known in outline to every schoolchild, it is 
different with the soil; not only because, unlike the 
atmosphere, soil varies much in composition and 
character, but also because the soil is in fact a very 
complex product, offering many difficult problems to 
the investigator; it is only of late years that the 
scientific study of the soil has been placed on a sound 
basis; our knowledge of it is still far from complete. 
Whence does soil arise? How is it that the surface 
of the land is usually covered with a layer of fertile 
