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A TEXTBOOK OF THEORETICAL BOTANY 



dichotomously divided. It is not until the fifth or sixth leaf is reached that 

 the pinnate form of the mature leaf appears. This young sporeling develops 

 into the ordinary Fern, and since it will later produce spores, it is spoken of 

 as the sporophyte generation. 



Primary root 



Second leaf 



First leaf 



Fig. 493. — Dryopteris filix-mas. Prothalli with attached embryos illustrating the 

 change from the palmate form of the earliest leav'es to the pinnate form of the 

 mature leaves. 



Alternation of Generations 



Since in the act of fertilization the chromosome number is doubled, the 

 Fern plant to which it gives rise will have nuclei containing the double or 

 diploid number of chromosomes. In the Fern the meiosis or halving of the 

 chromosome number takes place in the formation of the spores. Thus the 

 nuclei of all the cells of the prothallus and the sex organs and gametes borne 

 on it will have the single or monoploid number. It may be said, therefore, 

 that the Fern exhibits an alternation of monoploid and diploid generations. 



