INTRODUCTORY SURVEY 17 



Underlying the differences b}- which these various groups are 

 distinguished from one another, there are many fundamental 

 similarities in structure and function which are common to all 

 plants; but the marked changes which appear as we pass from 

 the lowest to the highest types make very difficult a concise 

 description of the characteristics of the plant kingdom as a whole. 

 It will therefore, be profitable for us to confine our attention, 

 at first, mainly to those plants which are most familiar to every- 

 one and of most obvious and immediate interest to man — the 

 Seed Plants. The sciences of morphology and physiology, 

 as exemplified by the Seed Plants, will accordingly be the chief 

 object of study in the first portion of this book. In these early 

 chapters we should be careful to remember that only one division 

 of the plant world — albeit the most important one — is being 

 considered, although many of the facts and principles there estab- 

 lished are, of course, valid for all plants. In the last chapters of 

 the text we shall discuss in some detail the lower members of the 

 vegetable kingdom, and the respects in which they differ from 

 each other and from the Seed Plants. 



The great variety of plant types and the diversity of condi- 

 tions under which they live renders it difficult to make general 

 statements about plants which are universally true, for exceptions 

 to any such statement may usually be found. Indeed, varia- 

 bility is one of the most notable characteristics of all life. The 

 student should therefore bear in mind that man}- of the facts 

 and principles set forth briefly and simply in an clementar}- text 

 are to be taken as true for typical cases and under ordinary con- 

 ditions, and he should be careful not to accept them as necessarily 

 and universally true for all plants and under all conditions. 

 Living things are too complicated to be described completely 

 in simple formulas. 



The Structures and Functions of Plants. — A notable character- 

 istic of plants, which they share with all other living things, is 

 their very definite bodily form. This form is not merely external 

 or accidental but is the mark of a fundamental organization or 

 "division of labor" within the plant itself. The individual is 

 made up of a series of distinct and visibly different parts, each 

 of which possesses a specific shape and performs a specific 

 function (Fig. 10). These parts are called organs. The root, 

 the stem, the leaf, the flower, the fruit, and the seed, as well as 



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