Chapter I 

 WHAT IS PLANT GEOGRAPHY? 



Let us begin with a few basic definitions and follow them with 

 some general explanations. 



Biology is the science of life, the study of living things, and it 

 has two main branches — botany, which deals with plants, and 

 zoology, which deals with animals. But whereas every one of us 

 must surely be clear about the differences between the typical plant 

 (which is static, green, and does not ingest solid food) and the 

 typical animal (which is motile, not green, and ingests elaborated 

 food), there remain many ' border-line cases ' that are apt to be 

 claimed by both botanists and zoologists. Indeed, each of the 

 characteristics mentioned for one of these primary groups (king- 

 doms) of living organisms is exhibited by some members of the 

 other, which prevents the drawing of any hard and fast line between 

 all animals and all plants. And even if we add the stipulation that 

 the greenness of plants shall be due to chlorophyll, and that they 

 shall contain the carbohydrate cellulose, there remain many organisms 

 which possess neither feature but still in other ways seem to be 

 plants, and are usually treated as such. 



Consequently it seems best in this case not to attempt precise 

 definition but rather to visualize the typical plant as a living organism 

 that is fixed, possessed of cellulose cell-walls, and dependent for its 

 main food-supply upon simple, gaseous or liquid substances (princip- 

 ally carbon dioxide and water). With the aid of chlorophyll in the 

 light, the organisfn builds up these substances into sugars and other 

 complex materials. The green plant is thus responsible for the 

 fundamental chain of reactions on which almost all life depends. 

 But this partial description excludes many organisms (such as Yeasts 

 and other small Fungi) which are commonly considered to be plants. 

 These ' exceptions ' often form major groups although, as we shall 

 see in the next chapter, they may exhibit none of the stipulated 

 main plant characteristics. The description also leaves behind a 

 basic ' hub ' of organisms, chiefly of microscopic types, that seem 

 to belong almost as much to one kingdom as to the other. Among 



