1g10] SHATTUCK—HETEROSPORY IN MARSILIA 31 
As there is nothing to interfere, the wall is laid down continuously over 
the entire surface of the cell, being thicker than the ordinary micro- 
spore wall, with no thin places, and absolutely unresponsive to ger- 
minative conditions. Cases of delayed or estopped germination due 
to resistant seed and spore coats are not uncommon, as shown by 
CROCKER (15) in the cocklebur and other plants. 
As said before, unfavorable conditions in the early life of the sporo- 
carp produced blasting of the megaspores, and sometimes delayed 
the reduction divisions in the 16 mother cells. However, unfavorable 
conditions at a later date permitted a very complex set of results, 
most of the megaspores blasting, while some of the microspore mother 
cells would undergo division and separate into spores (fig. 21). An 
interesting condition was obtained by blasting just as the micro- 
Spore tetrads were formed, and then transferring the plants to the 
best condition for growth. In this case a few microspores would 
survive in a few of the sporangia, growing with astonishing vigor, 
some of them reaching a size quite beyond the ordinary microspores 
(figs. 24-27). We have in this case an intermediate sized spore, 
sufficiently unlike the microspore to warrant the question whether 
the abortion of tetrads among the megaspores did not begin in this 
way. The difference in size between the ordinary microspores and 
these unusual ones is as great as that shown between the reputed 
microspores and megaspores of Calamostachys and Sphenophyllum. 
Probably the most interesting of all the results were obtained by 
growing Marsilia in subdued light during the usual time of sporocarp- 
formation (July and August), and then forcing the plants to develop 
sporocarps at a later period. I had previously determined that 
although the plants grew luxuriantly in subdued light, they would not 
produce sporocarps. On August 15, 1906, plants were moved to the 
south end of the greenhouse and given the maximum amount of 
heat and light. My intention in this experiment was to throw all 
the vigor of the plant into a few sporocarps and then to inhibit further 
Sporocarp-formation by means of a spray of cold water. A few sporo- 
carps appeared by September 10, which was more than two months 
later than the usual time. for sporocarp-formation. These grew with 
Steat vigor until the occurrence of exceptionally cool and cloudy 
Weather, which began September 19. After this time no more sporo- 
