THE MORPHOLOGY AND AFFINITIES OF GNETUM 1 43 
the parietal layers begin to separate from each other a cleft appears 
between them and the sporogenous tissue. The outermost layer of 
sporogenous cells then quickly takes on the characters of a tapetum, 
both cytoplasm and nuclei becoming very dense and two nuclei ap- 
pearing in each cell (figs. 9 and 10). In this case at least there can 
be no doubt that the tapetum is derived from sporogenous tissue. 
By the time the tapetum is fully differentiated the sporogenous cells 
within it have reached the mother-cell stage. The latter cells become 
more or less rounded and separated and then follow the tetrad di- 
visions which produce the pollen grains. These divisions are not 
simultaneous throughout the sporangium as in Angiosperms but 
within a single sac all stages may be found from the undivided mother- 
cell to the young spore (fig. 10). Both tetrad divisions are completed 
before walls are formed. The four pollen grains occupy only a small 
part of the space within the mother-cell. Before the tetrad divisions 
are completed the granular substance referred to is all that remains of 
the parietal cells, and the tapetum has also begun to disintegrate 
(fig. 10.) When the young pollen grains become free the whole sac 
consists of a single layered epidermis, the granular remains of tapetum 
and wall cells and the mass of young microspores. 
At maturity the stalk of the stamen elongates, breaking through 
the perianth and projecting beyond the collar of the strobilus. De- 
hiscence occurs by means of a cleft at the top of each sporangium. 
The first stamens to protrude are, of course, the uppermost ones. 
These soon fall and their places are taken by those next below and 
so on until all the stamens within the collar have matured. 
The period of time occupied by the events just described is much 
shorter than is usual in Gymnosperms. The whole course of develop- 
ment up to the shedding of the pollen takes place in a few weeks. 
The only Gymnosperm which approaches Gnetum in the rapidity of 
this development is Ephedra, another member of the Gnetales, in 
which according to Land (15) the staminate strobilus is first recog- 
nizable in December (in New Mexico) and the pollen is shed in April. 
Indeed the period of development is shorter than in even the spring- 
blooming Angiosperms in which as a rule the winter is passed in the 
mother-cell stage. The summer-blooming Angiosperms it seems are 
the only seed plants whose microsporangia develop so rapidly. 
