440 TEXTBOOK OF BOTANY 



one particular kind of plant. These thousands of differences are heritable 

 and are determined by something that is transmitted from parent to 

 offspring in each kind of plant. Then one may try to visualize some of 

 the thousands of successive events that must occur in approximately the 

 same manner and order every time an oak tree develops from an acorn. 

 These repeated similarities also are dependent upon something that is 

 transmitted from parent to offspring. 



If we can first secure acceptable scientific answers to such questions 

 as those listed above, we shall then have a much surer background of 

 fact by which to explain why the progeny resembles the parents in some 

 respects and differs from them in others, and why these resemblances 

 and differences appear in the definite proportions first described bv 

 Mendel. The answers will also help us to understand why there are 

 fewer variations when plants propagate only by vegetative means, why 

 plants do not always "come true from seed," and why inbreeding does 

 not always result in less desirable plants. 



In addition we may wish to apply our knowledge to a consideration 

 of questions of more general significance. What, for instance, is the ex- 

 planation of the occurrence of new kinds of plants in successive geologic 

 periods? How do new kinds of plants come to be? What are the causes 

 of evolution, and in what ways is it limited? 



In this and the next three chapters you will find a discussion of some 

 of the more basic facts necessary to arrive at reasonable answers to the 

 questions suggested above. For lack of space many interesting facts will 

 have to be omitted. If the reader is interested in knowing more about 

 some particular phenomenon, the books listed at the end of the chapter 

 will be helpful as sources of data and further references. 



Heredity in vegetative multiplication and cross-fertilization contrasted. 

 We may begin with the familiar fact that when a desirable plant is ob- 

 tained by means of cross-pollination it does not always reproduce true 

 from seeds, but it may multiply unaltered by vegetative propagation for 

 an indefinite period of time. In practice a large number of plants having 

 the same heredity may be obtained by means of continued vegetative 

 propagation, starting, of course, with cuttings all of which were taken 

 from the same plant. The cuttings, however, may be taken from the 

 roots, stems, leaves, or from any two or three of these organs. The indi- 

 viduals of a clone of plants obtained in this manner all have the same 

 heredity ( Chapter XXXV ) . This fact is evidence that all the cells of the 



