186 COMPENDIUM OF GENERAL BOTANY. 



the germs themselves and the plants proceeding therefi'om are 

 different. 



We can now recognize two categories of phenomena which may 

 both be observed on the same plant at different 2:)eriods. When 

 different methods of reproduction are united in the same plant- 

 individual, we are not concerned with alternation of generation. 

 If different methods of rej)roduction do not occur in the same indi- 

 vidual, but alternate with the successive generations of a plant, we 

 speak, in general, of alternation of generation. By alternation of 

 generation we therefore mean the unequal behavior of the succes- 

 sive generations of the same plant with regard to the mode of 

 reproduction. Concerning the first-mentioned phenomenon we 

 will not have much to say; considerable, however, in regard to 

 alternation of generation. 



Let A and B represent different methods of re^^roduction (for 

 example, sexual and asexual) ; they may be so distributed through 

 the generations 1, 2, 3, 4, 5, 6, etc., that 1, 3, 5, etc., are the result 

 of the method A\ 2, 4, 6, etc., of the method B. Of very fre- 

 quent occurrence is that form of alternation of generation in which 

 method B is common to a series of successive generations, while 

 method A occurs in only one generation ; then another series of 

 method B., etc. The following scheme will illustrate this : 



Series of Generations. 



1, 2, 3, 4.. .«, «-j-l, n-|-2, n-t-3, n-|-4, n-\-h ...n-\-m, n-\-m-\-\,n-\-ni-\-2... 



I I I I i I I I I I I I I 



BBBB B A B B , B B B A B B... 



Methods of Reproduction. 



In the case of alternation of generation the different forms of 

 reproduction are equal in value in so far as they are necessary to 

 the maintenance of the plant-species. The same may be said of all 

 forms of rej)roduction. When there is no alternation of genera- 

 tion, but simply a combination of different methods of I'eproduction 

 in the same individual, then these various methods are of unequal 

 value, because, as a nile, one form of reproduction shows itself to 

 be constant and more essential, and usually occurs at the conclusion 

 of development, while the others (non-essential) make their appear- 

 ance earlier. Example : a plant which iinally produces seeds sex- 

 ually, that is, from flowers, may in the cc^urse of its life-history be 



