DIMORPHISM A PHENOMENON OF ALTERNATIVE EXPRESSION. 9 



coiil'iision ol" the facts of expression witJi tliose of transmission. Tiie 

 phenomena of inheritance have been supposed to center exchisively 

 in the germ cells, the assumption being that all tlio characters that 

 are to be sliown in the adult are determmcd beforehand in the germ 

 cells. TJio facts of (limor{)hism suggest that the phenomena of 

 heredity find breeding can be studied in tlie vegetative parts of the 

 plants as well as in the floral or reproductive organs or in the proto- 

 plasmic mechanism of the germ cells. 



The production of a succession of different kinds of internode indi- 

 viduals by vegetative propagation shows that characters may be 

 brought into expression and then suppressed and replaced by other 

 characters without the necessity of new conjugations to form new 

 germ cells. In all the higher plants the expression of the characters 

 is changed repeatedly during the growth of each individual. This 

 may be one of the reasons why the processes of heredity appear to be 

 more susceptible to environmental influences in plants than in 

 animals. 



That the leaves and other vegetative parts of many plants do not 

 have the power of regenerating or bringing the characters of the 

 other parts into expression does not demonstrate a fundamental 

 difference between the germinal and somatic protoplasm. In some 

 plants, such as the Begonia, it is evident that all of the tissues inherit 

 all of the characters, since new plantlets are able to bud out freely 

 from the leaf blades, petioles, and stalks. 



In Bryophyllum also young plantlets are produced from the leaves, 

 but only from i)articular points along the margins instead of from 

 the whole surface of the leaf. But even with the most definite limi- 

 tations of expression there may be evidences of complete transmis- 

 sion. Thus lateral branches of coffee, though apparently quite 

 unable to produce upright shoots from vegetative buds, are certainly 

 able to transmit all the characters of the species, for all the fruit is 

 produced on the lateral branches. 



If there were a complete correspondence between expression and 

 transmission, so that the transmitted characters of a variety could 

 be fully known from a single individual or from a generation of 

 uniform individuals, the characters of a pure-bred uniform variety 

 might be expected to remain fix;ed for all time and further selection 

 would be entu'ely unnecessary, as assumed in some theories. But 

 in reality no such permanent uniformity has been found to exist. 

 No refinement of the breeder's art establishes an unchanging expres- 

 sion of characters in any seed-propagated plant, or even in those 

 that are increased by vegetative propagation. It is easy to under- 

 stand that selected strains of wheat or other plants adapted to self- 

 fertilization may show greater and more permanent uniformity than 

 95213°— Bui. 221—11 2 



