CHAPTER II 

 INTRODUCTORY SURVEY 



Before commencing an intensive study of any aspect of botani- 

 cal science or of any particular problem which deals with plants, 

 it will be well for us to make a brief survey of the plant kingdom 

 as a whole, and of some of the more important structures and 

 functions of plants in general. 



The Plant Kingdom. — About 250,000 different kinds or species 

 of plants have been discovered and described, and every year 

 botanical exploration and careful study bring more of them to our 

 knowledge. We have seen that the problem of systematic botany 

 is to name this host of plants and to arrange. and classify its 

 members in a logical system. Over many of the details of such 

 a classification difference of opinion still exists, but there is now 

 rather general agreement as to the main groups into which the 

 plant kingdom should be divided. Four such divisions are 

 commonly recognized : 



A. The Thallophytes. — These are lowl}- plants, various in their 

 structure, activities, and methods of reproduction, but agreeing 

 in the possession of a simple body without roots or leaves and in 

 multiplying by single-celled spores. The majority of Thallo- 

 phytes inhabit water or moist places and are small and soft-bodied 

 plants. 



There are two main series of Thallophytes: The Algae (Fig. 4), 

 which possess the green pigment cJdorophyll and are thereby able 

 to manufacture their own food, and which include all the seaweeds 

 and their fresh-water allies; and the Fungi (Fig. 5), which lack 

 chlorophyll and consequently are obliged to obtain their food 

 from hving animals and plants or from dead organic material. 

 Here belongs the vast array of bacteria, molds, blights, rusts, 

 toadstools, mushrooms, and similar plants, many of which live as 

 parasites and are often the cause of serious diseases of man and 

 the lower organisms. 



B. The Bnjophyics or Moss Plants. — These plants are dis- 

 tinguished from the Thallophytes chiefly by their more highly 



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