Nichols, A morphological study of Juniperus communis var. depressa. 219 



chromatic elements being heaped together in a compact mass which 

 lies close to the nuclear membrane. The cytoplasm of the cell 

 appears more or less alveolar, and in it are imbedded niimerous 

 minute starch grains. The megaspore motlier cell is slightly larger 

 than the microspore mother cell, raeasuring about 35 /u x 20 ju, 

 while its nucleus has a diameter of abont 17 /t. 



Tetrad formation takes place about April 20 th . The first 

 divisionof the mother cell nucleus, as observed by Noren (1907), 

 gives rise to two nuclei containing the haploid number of chromo- 

 somes. The spindle of this division is represented in figs. 61 and 

 67. In the latter figure it will be seen that a cell plate has been 

 formed, but a permanent membrane is rarely if ever laid down 

 between the daughter nuclei. In thisrespect the heterotypic divi- 

 sion of the megaspore mother cell resembles that of the microspore 

 mother cell. Frequently a protoplasmic membrane separates the 

 two nuclei, but often theylie free in the cytoplasm of the mother 



Fig. 2. 



Longitudinal section through young ovule about two weeks after pollination. 

 Micropyle elosed. X ^6. 



cell (fig. 63). Both daughter nuclei become more or less com- 

 pletely reorganized, but as a rule only the lower one of the two 

 reaches a resting stage. In such cases the latter alone undergoes 

 the homotypic division, and as a result there is usually produced 

 a group of three cells (fig. 67), only two of which are morpho- 

 logically megaspores. Where both nuclei divide, a true tetrad is 

 formed. Figs. 64 — 66 represent instances in the species studied 

 where both daughter nuclei are undergoing the homotypic division, 

 and where four potential megaspores are thus being developed. 

 It will be seen that the two spindles may lie either side by side 

 or in Tandem'. No especial attention was given to the formation 

 of these spindles, but fig. 64 shows a multipolar diarch similar to 

 that noted by Miss Ferguson (1904, fig. 142) in Pinus. 



Noren (1907) states that he has never observed a case 

 where more than one embryo sac has developed within a Single 

 nucellus. The writer, however, was fortunate enough to secure 

 one preparation in which three megaspore mother cells had under- 



