646 PHARMACEUTICAL BOTANY 
10 
Fic. 484.—Diagrams illustrating the behavior of chromosomes during the reduc- 
tion division which occurs at the time of spore formation in ferns and in most other 
plants which have sexual reproduction. The maternal chromosomes are shown in 
black, the paternal in white. Homologous chromosomes (corresponding maternal 
and paternal chromosomes) are of corresponding length. As a result of the process 
here shown, the spore mother cell (in the resting condition in 1) gives rise to a tetrad 
or group of 4 spores as shown in 10, The spore mother cell nucleus (1) has the 
diploid chromosome number (in this case 6) which is characteristic of the cells of the 
sporphyte. These six consist of 3 pairs of homologous chromosomes, one of each 
pair having come from the male gamete and one from the female gamete in the last 
fusion of gametes. Note that in 2 the three pairs with the chromosomes of each 
pair in close contact are lying within the nuclear membrane. In 3 the paired 
chromosomes have shortened and have become closely “‘knotted” whereas in 4 they 
have become “unraveled” and have undergone further shortening. In 5 the mem- 
bers of each pair have separated but are still close to one another. In 6 the nuclear 
membrane and the nucleolus have disappeared and the six chromosomes, each 
showing evidence of a later lengthwise splitting, have been separated into two groups 
of three. This is the actual reduction division, Note that in 7, only one chromosome of 
each homologous pair is present in each of the resulting nuclei which are therefore 
different in the chromosome “stock.” Each of these nuclei now undergoes a second 
or homotypic division (8) with a splitting of each of the chromosomes, so that there 
are two kinds of nuclei in the tetrads (shown in 9), two having the same chromosomes 
as the upper nucleus in 7 and two having the same chromosomes as the lower nucleus 
shown in 7. (From ‘A Text Book of General sy ‘ail Rebicus by peril 
sion of the Publishers, J. Wiley @ kg me) Botany” by Holman and Robbins by per 
