IgIo] SHATTUCK—HETEROSPORY IN MARSILIA 35 
spores in certain sporangia; then better nutrition caused the excessive 
development of the survivors at the expense of the aborting ones; 
and this process continued until specialized spores (megaspores) were 
formed. 
Heterospory, as found in Calamostachys, is in its incipiency. We 
See its very beginnings in C. Binneyana, where all the sporangia show 
a tendency toward it by the promiscuous abortion of some members 
of all the tetrads, with the corresponding enlargement of the survivors. 
In C. Casheana we see it feebly established, abortion being confined 
to certain sporangia, yet fully three-fourths of them showing abort- 
ive spores quite variable in size. 
In Marsilia, in ordinary plants, heterospory is much more firmly 
established, abortion being confined to not more than one-fifth or one- 
sixth of the sporangia, and at maturity the megaspores are very nearly 
equal in size,and many times larger than the microspores. But when by 
any means the fertility of these megaspores is destroyed, there is stilla 
very large number of potential spores in the original homosporous con- 
dition and sufficiently young to respond to the stimulus given them 
by the added nutrition which should have gone toward maturing the 
megaspores. We then see the plant repeating its probable ancient 
habit of developing some of the spores to greater size at the expense 
of the others; and we have repeated in every detail the processes 
Observed in C. Binneyana, which is homosporous. Again, well- 
nourished plants, allowed to produce only a limited number of sporo- 
Carps, carry this abortion and enlargement still farther, and a few 
Sporangia produce still larger spores, varying as much in size as in 
C. Casheana, which is heterosporous. Figs. 32-43 show that in many 
sporangia abortion and enlargement continue much farther than in 
C. Casheana, in that an intermediate sized megaspore is produced. 
This adds more evidence as to the plasticity of the plant and the man- 
her in which heterospory may have originated in it. 
Summary 
I. It is possible by means of a spray of cold water to kill the mega- 
Spores of Marsilia, which occur only in the oldest sporangia, and then 
by putting the plant under good conditions to mature sporocarps 
Without megaspores. 
