TgIo} SHATTUCK-—-HETEROSPORY IN MARSILIA 2I 
in which I could not find a single plant bearing archegonia, though 
plants bearing antheridia were very numerous. This shows that 
Equisetum is only slightly more advanced in its. tendency to produce 
dioecious prothallia than the ferns, and that this tendency is largely, 
if not wholly, controlled by external conditions after the spore begins 
to germinate, and probably not until the prothallium consists of many 
cells. This has also been mentioned by PRANTL (21) and others. 
Varied external conditions operating on the plant during the formation 
and maturation of the spore seem to make no difference in its size or 
tendency after germination to produce male or female prothallia. 
In other words, the plant makes no preparation in its spore for the 
production of female prothallia, the sex being determined long after 
germination and wholly by chance external conditions. 
From a consideration of these facts it appears that the genus 
Equisetum, as we now know it, furnishes no clue to the origin of 
heterospory, because it makes no preparation which we can discover 
in the organization of its spores, either in size, shape, or extra storage 
of food material for the production of specially large gametophytes. 
In other words, the sex of the prothallium seems to be determined after 
germination and wholly by external conditions. In this particular 
we find it no farther advanced than the true ferns. 
When we compare this form with one of the heterosporous pterido- 
phytes, for example Marsilia, we find that the latter plant begins to 
prepare a few special spores as soon as the tetrad divisions have 
occurred, which are to produce the female prothallia. These few 
large spores are produced at the expense of many aborting ones whose 
Substance is gradually absorbed by them. Moreover, these large 
Spores are formed, under normal conditions, in the oldest and most 
favorably placed sporangia (figs. 1, 2), thus showing that here the 
Sporophyte makes definite preparation for the production of spores 
which are to produce female prothallia. 
In the microsporangium every one of the 16 mother cells (jig. 3) 
produces four functioning microspores, 64 in all; while in the mega- 
‘porangium only one spore matures out of 64, all of which seem to be 
identical in every particular when the tetrads are first formed (fig. 4)- 
A closer examination reveals the fact that this one megaspore does 
not gain the ascendency without a struggle. In fact, many instances 
