CONDITIONS OF GAMETE FORMATION 369 



differentiation of the plant body. Both morphologically and 

 physiologically it is a much more highly differentiated and special- 

 ized system than the vegetative axes of the plant. 



This being the case, we should expect to find the flower as the 

 final stage of development, as the expression of maturity of the 

 plant. Among the flowering plants this appears in general to be 

 the case. The young plant grows, produces new vegetative axes, 

 and in most cases becomes what the zoologist would term an asexual 

 colony, but after a longer or shorter period of such vegetative growth 

 and reproduction, varying in different plants from a few weeks to 

 many years, flower buds appear in place of certain or all of the 

 vegetative buds, gametes are produced, and seeds are formed. 

 In many plants vegetative growth ceases when flowering occurs, 

 and flowering is followed by death of the whole plant, except the 

 seeds, but in others the sequence may be repeated an indefinite 

 number of times during the life of the plant. 



To all appearances then these plants have a definite life history, 

 vegetative growth and reproduction of vegetative axes being 

 characteristic of the earlier stages and the development of flowers 

 and gametic reproduction of the later stages. In those plants 

 where the sequence is repeated periodically, different shoots or axes, 

 that is, different plant individuals, are concerned in each period. 

 Moreover, it is a well-known fact that in general cuttings from 

 plants in bloom or ready to bloom are likely to bloom earlier than 

 cuttings from plants which are still in the stage of active vege- 

 tative growth. Such facts indicate clearly that flowering is an 

 expression of internal conditions which are characteristic of a rela- 

 tively advanced stage in the life of the plant or in a seasonal or 

 other period of metabolic activity and growth. 



But certain facts of observation and experiment have often been 

 regarded as pointing to a somewhat different conclusion. First 

 among these is the familiar fact, to which attention has already been 

 called (pp. 239-44), that many plants live and grow indefinitely 

 without sexual reproduction. This is true, not only of many 

 rhizome plants, in which the rhizome or rootstock grows con- 

 tinuously and produces new buds and roots, and from time to 

 time branches, while at the other end death continually advances, 



