4 8 



ALTERNATING GENERATIONS 



latter the " haploid " phase. It happens that the facts are more readily 

 observed in most Seed-Plants than in Pteridophytes, owing partly to the 

 greater size of the spindles, partly to the number of the chromosomes 

 being smaller than in most of the Archegoniates. Accordingly we have 

 the record of the numbers from over 60 Seed-Plants, showing with remarkable 







FIG. 31. 



Successive stages of nuclear- and cell-division in an embryonic tissue. n = nucleus. 

 /=nucleolus. w==nuclear wall. c = cytoplasm. ch = chromosomes. = polar-cap. 

 s spindle. kt> = nuclear plate. / = young daughter-nuclei. v connecting threads. 

 z = cell-plate. 7w = nevv septum. In i the resting nucleus is shown. In 2 and 3 the 

 segregation of the chromosomes. In 4 the chromosomes are seen with transverse discs. 

 In 5 the arrangement of the chromosomes to form the nuclear plates and their longitudinal 

 fission. In 3-5 the formation of the spindle from the polar-caps. In 6 the longitudinal 

 fission of the chromosomes. In 7 their separation towards the poles has begun. In 8 the 

 daughter-chromosomes are completely separated. In 9 they are proceeding towards 

 the poles. In 10, u, and 12 the daughter nuclei are being formed. In 9-11 the connecting 

 threads and the cell-plate are being formed. In 12 the completion of the septum. 

 X about 600. (After Strasburger.) 



constancy that the number of chromosomes in the dividing nuclei of the 

 sporophyte is double that in those of the gametophyte. Common numbers 

 are 32 : 16 24 : 12 16 : 8; while they run as low as 12 : 6 or 6 : 3, and 

 as high as 64:32 or 96:48. 



Records from the Gymnosperms bring evidence of the same difference 

 between the two generations in various species of Pinus, etc. Among 

 Pteridophytes it has been observed in such Ferns as Osmunda, Ahophila, 



