I9Io] SHATTUCK—HETEROSPORY IN MARSILIA 33 
functionless tetrads, which, while now distorted and shriveled, must 
have drawn heavily upon the supply of nutrition. This may explain 
why the largest microspores, while acquiring certain megaspore charac- 
ters, were not able to attain more fully to megaspore proportions. In 
examining the sporangia that contained secondary megaspores, I 
was never able to find these large shriveled tetrads. This shows that 
all the tetrads must have aborted during the early stages of develop- 
ment, thus permitting the whole of the nutrition to be concentrated 
in this one spore, as is the usual case in primary megasporangia. 
Fig. 36 is unusual for secondary megaspores, and shows in addition 
to the successful spore a single member in each of the three tetrads 
slightly enlarging. Fig. 37 is more usual, and shows five tetrads 
which aborted much earlier. Two cultures were grown in 1907 under 
conditions similar to those just described, and the same results were 
obtained. Occasionally in all of these cultures I found megasporangia 
containing well-developed microspores (figs. 2, 33, 35, 37). In ordi- 
nary plants these were always found on the periphery of the sporo- 
genous area and appeared to be related to the food supply. Scant 
nutrition may cause the sporogenous tissue in some megasporangia 
to revert to the probable ancient homosporous condition. 
This production of secondary megaspores I believe to be as closely 
related to nutrition as is the formation of primary megaspores. In 
numerous cultures I was able to observe that the formation of mega- 
Spores is possible only when the plants are well nourished. In the 
cultures in Which I secured secondary megaspores, I always had long 
and strong rhizomes and allowed only a few sporocarps to form just » 
at the end of or after the fruiting season. Then by cutting off all 
later sporocarps I was able to throw all the energies of the plant into a 
very few. By this means I was able to secure not only the most 
marked abortion and enlargement of the microspores, but also the 
formation of secondary megaspores, which, since they occur in micro- 
sporangia, would undoubtedly have developed, under ordinary con- 
ditions, as microspores. 
tn a plant so plastic as Marsilia, it is not difficult for one to con- 
ceive how the heterosporous habit may have become fixed. If exces- 
sive nutrition will now cause certain microsporangia to develop 
megaspores, and excessively hard conditions will cause the plant to 
