48 DIMORPHIC LEAVES IN RELATTOX TO HEREDITY. 



of the progeny. Or beards might be considered as not inherited 

 because they are not developed in chiklren. In all such cases there 

 is a temporary latency or postponement in the expression of charac- 

 ters, but no failure of inlieritance in the sense of transmission. The 

 adult characters remain latent during the larval or juvenile stages, 

 and the juvenile characters are suppressed in turn during the adult 

 stages. In the development of each individual plant several such 

 changes in the expression of the characters are regularly required for 

 the formation of the different kinds of vegetative and reproductive 

 organs. 



In the cotton plant six different forms of leaf organs may be recog- 

 nized, the cotyledons, the entire or broad-lobed leaves at the base of 

 the stalk, the more divided leaves farther up, the smaller, narrower 

 leaves of the fruiting branches, and the two stiU more reduced and 

 specialized forms that compose the outer and inner involucres. To 

 form the petals, stamens, and pistils requires three other changes of 

 characters, making nine changes altogether during the course of 

 development of each plant. 



The familiarity of the facts makes an adequate appreciation dif- 

 ficult, but if the individuality of the internodes and their method of 

 development, one after another, be recognized, it becomes plain that 

 the changes of characters that take place during the growth of the 

 plant are much more profoimd than those that are required in the 

 postembryonic development of an animal. The whole complex of 

 characters expressed in one internode individual may give place to 

 the expression of an entirely different complex in the very next 

 internode. Without any opportimity for new conjugations, segre- 

 gations of characters in different germ cells, or changes in the numbers 

 of chromosomes, one complex of characters after another is called into 

 expression and the previous complex retired to a latent condition. 



Failure to effect the full change of expression results in the devel- 

 opment of abnormal organs of intermediate form, as in the case of 

 abnormal intermechate branches in the cotton phint. Sucli branches 

 are usually sterile, or their flower buds are abortive, as in abnormal 

 hybrids or hermaphrodites. The power to com])lete the various 

 alternations in the expression of the characters determines the possi- 

 bilities of development in the individual plant, in the same way that 

 the evolutionary progress of a species is determined by evolutionary 

 changes of characters. 



The phenomena of alternative expression have been studied largely 

 from the standpoints of environmental moiUfications and (Hversi- 

 ties in hybrids. These groups of phenomena are only a small part of 

 the field of alternative expression, which inchides also the endless 

 changes of characters that appear during the ordinary processes of 



221 



